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At least 559 records · Page 31Linked to original sources
USGS2005

Baseline water-quality characteristics of the Alaska Army National Guard Stewart River Training Area near Nome, Alaska

The Alaska Army National Guard Stewart River Training Area is approximately 23 miles north of Nome on the Seward Peninsula in northwest Alaska. The Stewart River Training Area encompasses much of the Stewart River Basin and a small part of the Snake River Basin. Hydrologic, water-quality, and physical-habitat data were collected at seven surface-water sites within the Stewart River Training Area during the summer runoff months (late-May to early-September) in 2004. Two of the sampling sites selected for this study were on the main stem Stewart River, one at the upstream boundary and one at the downstream boundary of the training area. Continuous hydrologic, precipitation, and water temperature data were collected at these two sites throughout the summer of 2004. Three pond sites, along the upper, middle, and lower reaches of the Stewart River within the training area, were each sampled twice during the summer of 2004 for analysis of water-quality constituents. Two tributaries to the Snake River Basin, Goldbottom Creek and North Fork Snake River, within the Stewart River Training Area boundary, also were sampled twice during the summer of 2004. Water-quality data collected from the Stewart River at the upstream and downstream study sites indicate similar constituent concentrations. Concentrations of most water-quality constituents collected during the summer of 2004 did not exceed standards for drinking water or recreational contact. Analysis of trace-element concentrations in bed sediment samples indicate the threshold effect concentration (below which no adverse effects on organisms is expected) was exceeded for arsenic, chromium, and nickel concentrations at all sample sites within the Stewart River Training Area and cadmium, copper, zinc, and lead concentrations were found to exceed the threshold effect concentration in varying degrees at the sample sites. The probable effect concentration (above which toxic effects on organisms is likely) was exceeded by arsenic concentrations at all sites except the lower pond site. Chromium and nickel concentrations exceeded the probable effect concentration at the upstream Stewart River site and at the North Fork Snake River site.

Scientific Investigations Report↗
USGS2008

Hydrologic and water-quality characterization and modeling of the Onondaga Lake Basin, Onondaga County, New York

Onondaga Lake in Onondaga County, New York, has been identified as one of the Nation’s most contaminated lakes as a result of industrial and sanitary-sewer discharges and stormwater nonpoint sources, and has received priority cleanup status under the national Water Resources Development Act of 1990. A basin-scale precipitation-runoff model of the Onondaga Lake basin was identified as a desirable water-resources management tool to better understand the processes responsible for the generation of loads of sediment and nutrients that are transported to Onondaga Lake. During 2003–07, the U.S. Geological Survey (USGS) developed a model based on the computer program, Hydrological Simulation Program–FORTRAN (HSPF), which simulated overland flow to, and streamflow in, the major tributaries of Onondaga Lake, and loads of sediment, phosphorus, and nitrogen transported to the lake. The simulation period extends from October 1997 through September 2003. The Onondaga Lake basin was divided into 107 subbasins and within these subbasins, the land area was apportioned among 19 pervious and impervious land types on the basis of land use and land cover, hydrologic soil group (HSG), and aspect. Precipitation data were available from three sources as input to the model. The model simulated streamflow, water temperature, concentrations of dissolved oxygen, and concentrations and loads of sediment, orthophosphate, total phosphorus, nitrate, ammonia, and organic nitrogen in the four major tributaries to Onondaga Lake–Onondaga Creek, Harbor Brook, Ley Creek, and Ninemile Creek. Simulated flows were calibrated to data from nine USGS streamflow-monitoring sites; simulated nutrient concentrations and loads were calibrated to data collected at six of the nine streamflow-monitoring sites. Water-quality samples were collected, processed, and analyzed by personnel from the Onondaga County Department of Water Environment Protection. Several time series of flow, and sediment and nutrient loads were generated for known sources of these constituents, including the Tully Valley mudboils (flow and sediment), Otisco Lake (flow and nutrients), the Marcellus wastewater-treatment plant (flow and nutrients), and springs from carbonate bedrock (flow). Runoff from the impervious sewered areas of the City of Syracuse was adjusted for the quantity that was treatable at the county wastewater-treatment plant; the excess flows were routed to nearby streams through combined-sanitary-and-storm-sewer overflows. The mitigative effects that the Onondaga Reservoir and Otisco Lake were presumed to have on loads of sediment and particulate constituents were simulated by adjustment of parameter values that controlled sediment settling rates, deposition, and scour in the reservoir and lake. Graphical representations of observed and simulated data, and relevant statistics, were compared to assess model performance. Simulated daily and monthly streamflows were rated “very good” (within 10 percent of observed flows) at all calibration sites, except Onondaga Creek at Cardiff, which was rated “fair” (10–15 percent difference). Simulations of monthly average water temperatures were rated “very good” (within 7 percent of observed temperatures) at all sites. No observed data were available by which to directly assess the model’s simulation of suspended sediment loads. Available measured total suspended solids data provided an indirect means of comparison but, not surprisingly, yielded only “fair” to “poor” ratings (greater than 30 percent difference) for simulated monthly sediment loads at half the water-quality calibration sites. Simulations of monthly orthophosphate loads ranged from “very good” (within 15 percent of measured loads) at three sites to “poor” (greater than 35 percent difference) at one site; simulations of ammonia nitrogen loads ranged from “very good” at one site to “fair” (25–35 percent difference) at two sites. Simulations of monthly total phosphorus, nitrate, and organic nitrogen loads were generally rated “very good” at all calibration sites. Sources of uncertainty in model results were identified, including (1) errors in precipitation data, (2) limitations in model structure, (3) nonuniqueness of values for highly sensitive parameters, (4) errors or bias in data used to calibrate the different components of the model, (5) misclassification of land-use and land-cover data, (6) changes in land use during the simulation period, (7) unidentified sources or sinks of chemical loads and water-quality processes that varied over time, and (8) differences in scale between large calibrated subbasins and small subbasins to which calibrated parameter values were transferred. Uncertainty in simulations of water-quality constituents was compounded by uncertainty in the processes on which the water-quality simulations were based. Therefore, sediment simulations were affected by uncertainty in the simulation of hydrology, and nutrient simulations were affected by uncertainty in both the hydrologic and sediment processes, as well as, in simulations of water temperature and dissolved oxygen concentrations. The calibrated model can be used to simulate scenarios that represent planned or hypothetical development and implementation of best-management practices in the Onondaga Lake basin and to assess the effects that these changes and practices are likely to have on rural and urban nonpoint sources of pollution to Onondaga Lake. Model results also can be used as input to a hydrodynamic model of Onondaga Lake that is being developed by Onondaga County and to prioritize areas of the basin where mitigative measures to decrease sediment and nutrient loads could provide the greatest benefits to Onondaga Lake.

New York↗
USGS2012

Evaluating lake stratification and temporal trends by using near-continuous water-quality data from automated profiling systems for water years 2005-09, Lake Mead, Arizona and Nevada

The U.S. Geological Survey, in cooperation with the National Park Service and Southern Nevada Water Authority, collected near-continuous depth-dependent water-quality data at Lake Mead, Arizona and Nevada, as part of a multi-agency monitoring network maintained to provide resource managers with basic data and to gain a better understanding of the hydrodynamics of the lake. Water-quality data-collection stations on Lake Mead were located in shallow water (less than 20 meters) at Las Vegas Bay (Site 3) and Overton Arm, and in deep water (greater than 20 meters) near Sentinel Island and at Virgin and Temple Basins. At each station, near-continual depth-dependent water-quality data were collected from October 2004 through September 2009. The data were collected by using automatic profiling systems equipped with multiparameter water-quality sondes. The sondes had sensors for temperature, specific conductance, dissolved oxygen, pH, turbidity, and depth. Data were collected every 6 hours at 2-meter depth intervals (for shallow-water stations) or 5-meter depth intervals (for deep-water stations) beginning at 1 meter below water surface. Data were analyzed to determine water-quality conditions related to stratification of the lake and temporal trends in water-quality parameters. Three water-quality parameters were the main focus of these analyses: temperature, specific conductance, and dissolved oxygen. Statistical temporal-trend analyses were performed for a single depth at shallow-water stations [Las Vegas Bay (Site 3) and Overton Arm] and for thermally-stratified lake layers at deep-water stations (Sentinel Island and Virgin Basin). The limited period of data collection at the Temple Basin station prevented the application of statistical trend analysis. During the summer months, thermal stratification was not observed at shallow-water stations, nor were major maxima or minima observed for specific-conductance or dissolved-oxygen profiles. A clearly-defined thermocline and well-defined maxima and minima in specific-conductance and dissolved-oxygen profiles were observed at deep-water stations during the summer months. Specific-conductance maxima were likely the result of inflow of water from either the Las Vegas Wash or Muddy/Virgin Rivers or both, while the minima were likely the result of inflow of water from the Colorado River. Maxima and minima for dissolved oxygen were likely the result of primary productivity blooms and their subsequent decay. Temporal-trend analyses indicated that specific conductance decreased at all stations over the period of record, except for Las Vegas Bay (Site 3), where specific conductance increased. Temperature also decreased over the period of record at deep-water stations for certain lake layers. Decreasing temperature and specific conductance at deep-water stations is the result of decreasing values in these parameters in water coming from the Colorado River. Quagga mussels (Dreissena rostriformis bugensis), however, could play a role in trends of decreasing specific conductance through incorporation of calcite in their shells. Trends of decreasing turbidity and pH at deep-water stations support the hypothesis that quagga mussels could be having an effect on the physical properties and water chemistry of Lake Mead. Unlike other stations, Las Vegas Bay (Site 3) had increasing specific conductance and is interpreted as the result of lowering lake levels decreasing the volume of lake water available for mixing and dilution of the high-conductance water coming from Las Vegas Wash. Dissolved oxygen increased over the period of record in some lake layers at the deep-water stations. Increasing dissolved oxygen at deep-water stations is believed to result, in part, from a reduction of phosphorus entering Lake Mead and the concomitant reduction of biological oxygen demand.

Arizona;California;Nevada;Utah↗
USGS2014

Streamflow, water quality, and aquatic macroinvertebrates of selected streams in Fairfax County, Virginia, 2007-12

Efforts to mitigate the effects of urbanization on streams rely on best management practices (BMPs) that are implemented with the intent of reducing and retaining stormwater runoff. A cooperative monitoring effort between the U.S. Geological Survey and Fairfax County, Virginia, was initiated in 2007 to assess the condition of county streams and document watershed-scale responses to the implementation of BMPs. Assessment of the data collected during the first 5 years of this monitoring program focused on characterizing the hydrologic and ecological condition of 14 monitored streams. Hydrologic, chemical, and macroinvertebrate community conditions in the streams monitored were found to be consistent, overall, with conditions commonly observed in urban streams. Hydrologically, the monitored streams were found to be flashy, with flashiness positively related to road cover in the watershed. Typical pH values of streams throughout the network centered around neutrality (pH = 7) with strong daily fluctuations apparent in the continuous data. Patterns in specific conductance were largely representative of anthropogenic disturbances—watersheds having the greatest percentage of open space and estate residential land-use had the lowest typical specific conductance values, and specific conductance variability was less than what is observed in watersheds that are more intensively developed. In watersheds having greater road coverage, and more development in general, increases in specific conductance over several orders of magnitude were observed during winter months as a result of the application of de-icing salts on impervious surfaces. Dissolved oxygen conditions were typically within the range required to support healthy biological communities, although occasional departures during summer months at some sites fell below the impairment threshold for streams in Virginia. Nitrogen (N) and phosphorus (P), concentration patterns were largely consistent across the network, with few exceptions. Nitrogen concentrations in monthly samples were generally low and dominated by nitrate. Exceptions to the generally low N concentrations occurred at Captain Hickory Run, which had a median total N concentration of approximately 4.9 milligrams per liter (mg/L), compared to the network-wide median of approximately 1.7 mg/L, and at Popes Head Creek Tributary, where total N concentrations spiked to 6–8 mg/L during low-flow periods in August or September of each year. Phosphorus concentrations in monthly samples were generally low and dominated by the dissolved fraction. Two monitoring stations in the network, Flatlick Branch and Frog Branch, are notable for having median total P concentrations that were, on average, approximately three times greater than the median total P concentration of 0.02 mg/L observed at the other 12 stations in the network. Suspended-sediment and nutrient loads and yields were similar to those of urbanized watersheds in other studies, although the yields from these urbanized basins were greater than, or within, the upper quartile of yields observed throughout the Chesapeake Bay watershed. Annual suspended-sediment loads ranged from 289–10,275 tons, with a median of 1,311 tons, and corresponding yields ranged from 107–2,827 tons per square mile (ton/mi 2 ), with a median of 277 ton/mi 2 . Annual total N loads ranged from 8,014–36,413 pounds, with a median of 21,314 pounds, and corresponding yields ranged from 3,361–8,360 pounds per square mile (lb/mi 2 ), with a median of 6,200 lb/mi 2 . Annual total P loads ranged from 380–6,558 pounds, with a median of 1,874 pounds, and corresponding yields ranged from 140–1,562 lb/mi 2 , with a median of 543 lb/mi 2 . Benthic macroinvertebrate community metrics indicated that streams throughout Fairfax County are generally of poor health. One station, Castle Creek, was an exception with results indicating relatively high quality aquatic health. Six additional monitoring stations were added to the network in 2012 to improve spatial coverage throughout Fairfax County. Monitoring activities are expected to continue at all 20 stations for the foreseeable future as BMP implementation is conducted.

Virginia↗
USGS2021

Hydraulic modeling at selected dam-removal and culvert-retrofit sites in the northeastern United States

Aquatic connectivity projects, such as removing dams and modifying culverts, have substantial benefits. The restoration of natural flow conditions improves water quality, sediment transport, aquatic and riparian habitat, and fish passage. These projects can also decrease hazards faced by communities by lowering water-surface elevations of flood waters and by removing the risk of dam breaches associated with aging or inadequate infrastructure. This report documents and provides results of one- and two-dimensional hydraulic models developed for selected rivers and streams in the northeastern United States where a dam was removed or a culvert was retrofitted. The models were developed for conditions before and after the dam removal or culvert modification. The discharges applied in the models included monthly discharges and flood discharges for the annual exceedance probabilities of 50, 20, 10, 4, 2, 1, 0.5, and 0.2 percent. This study, by the U.S. Geological Survey in cooperation with the U.S. Fish and Wildlife Service, demonstrates the benefits resulting from dam removal and retrofitting undersized culverts in terms of decreased water-surface elevations during flooding and improved fish passage. The U.S. Army Corps of Engineers Hydrologic Engineering Center’s River Analysis System was used to model the sites in one- and two-dimensional hydraulics, and decreases in the 1-percent annual exceedance probability discharge water-surface elevation were found at all sites studied. The decreases in water-surface elevation at sites in which the impoundment was removed ranged from 1.3 to 10.4 feet. One site, Bradford Dam in Westerly, Rhode Island, had only a 0.2-foot decrease, but at that site the dam was replaced by a series of weirs to retain the upstream impoundment and allow fish passage. Minimal differences were found between the water-surface elevations computed by the one- and two-dimensional models. The two-dimensional models, however, provide the additional benefit of detailed velocity and depth data throughout the channel at a resolution not possible with a one-dimensional model. These velocity and depth data allowed for assessment of the suitability for fish passage at the sites. Fish passage was improved at all the sites by removing the dams and retrofitting the culvert. Prolonged swim velocity criteria for selected fish species were maintained throughout three of the nine study sites, and burst swim velocity criteria were met at all study sites.

Connecticut, Massachusetts, New Jersey, Rhode Isla↗
USGS2009

Simulation of streamflow and water quality in the Leon Creek watershed, Bexar County, Texas, 1997-2004

The U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers and the San Antonio River Authority, configured, calibrated, and tested a Hydrological Simulation Program ? FORTRAN watershed model for the approximately 238-square-mile Leon Creek watershed in Bexar County, Texas, and used the model to simulate streamflow and water quality (focusing on loads and yields of selected constituents). Streamflow in the model was calibrated and tested with available data from five U.S. Geological Survey streamflow-gaging stations for 1997-2004. Simulated streamflow volumes closely matched measured streamflow volumes at all streamflow-gaging stations. Total simulated streamflow volumes were within 10 percent of measured values. Streamflow volumes are greatly influenced by large storms. Two months that included major floods accounted for about 50 percent of all the streamflow measured at the most downstream gaging station during 1997-2004. Water-quality properties and constituents (water temperature, dissolved oxygen, suspended sediment, dissolved ammonia nitrogen, dissolved nitrate nitrogen, and dissolved and total lead and zinc) in the model were calibrated using available data from 13 sites in and near the Leon Creek watershed for varying periods of record during 1992-2005. Average simulated daily mean water temperature and dissolved oxygen at the most downstream gaging station during 1997-2000 were within 1 percent of average measured daily mean water temperature and dissolved oxygen. Simulated suspended-sediment load at the most downstream gaging station during 2001-04 (excluding July 2002 because of major storms) was 77,700 tons compared with 74,600 tons estimated from a streamflow-load regression relation (coefficient of determination = .869). Simulated concentrations of dissolved ammonia nitrogen and dissolved nitrate nitrogen closely matched measured concentrations after calibration. At the most downstream gaging station, average simulated monthly mean concentrations of dissolved ammonia and nitrate concentrations during 1997-2004 were 0.03 and 0.37 milligram per liter, respectively. For the most downstream station, the measured and simulated concentrations of dissolved and total lead and zinc for stormflows during 1993-97 after calibration do not match particularly closely. For base-flow conditions during 1997-2004 at the most downstream station, the simulated/measured match is better. For example, median simulated concentration of total lead (for 2,041 days) was 0.96 microgram per liter, and median measured concentration (for nine samples) of total lead was 1.0 microgram per liter. To demonstrate an application of the Leon Creek watershed model, streamflow constituent loads and yields for suspended sediment, dissolved nitrate nitrogen, and total lead were simulated at the mouth of Leon Creek (outlet of the watershed) for 1997-2004. The average suspended-sediment load was 51,800 tons per year. The average suspended-sediment yield was 0.34 ton per acre per year. The average load of dissolved nitrate at the outlet of the watershed was 802 tons per year. The corresponding yield was 10.5 pounds per acre per year. The average load of lead at the outlet was 3,900 pounds per year. The average lead yield was 0.026 pound per acre per year. The degree to which available rainfall data represent actual rainfall is potentially the most serious source of measurement error associated with the Leon Creek model. Major storms contribute most of the streamflow loads for certain constituents. For example, the three largest stormflows contributed about 64 percent of the entire suspended-sediment load at the most downstream station during 1997-2004.

Texas↗
USGS2012

Simulation of streamflow, evapotranspiration, and groundwater recharge in the middle Nueces River watershed, south Texas, 1961-2008

The U.S. Geological Survey—in cooperation with the U.S. Army Corps of Engineers, Fort Worth District; City of Corpus Christi; Guadalupe–Blanco River Authority; San Antonio River Authority; and San Antonio Water System— configured, calibrated, and tested a watershed model for a study area consisting of about 7,726 square miles of the middle Nueces River watershed in south Texas. The purpose of the model is to contribute to the understanding of watershed processes and hydrologic conditions in the middle Nueces River watershed. The model simulates streamflow, evapotranspiration, and groundwater recharge by using a numerical representation of physical characteristics of the landscape and meteorological and streamflow data. Model simulations of streamflow, evapotranspiration, and groundwater recharge were performed for various periods of record depending upon available gaged data for input and comparison, starting as early as 1961. Because of the large size of the study area, the middle Nueces River watershed was divided into eight subwatersheds, and separate Hydrological Simulation Program—FORTRAN models were developed for each subwatershed. Simulation of the overall study area involved running simulations in downstream order. Output from the model was summarized by subwatershed, point locations, stream and reservoir reaches, and the Carrizo– Wilcox aquifer outcrop area. Four long-term U.S. Geological Survey streamflow-gaging stations were used for streamflow model calibration and testing with data from 1990 to 2008. Monthly evaporation estimates from 2001 to 2008 and waterlevel data from 1961 to 2008 at Lake Corpus Christi also were used for model calibration. Additionally, evapotranspiration data for 2006–8 from a U.S. Geological Survey meteorological station in Medina County were used for calibration. Streamflow calibrations were considered poor to very good. The 2000–8 calibration results were characterized as good to very good for total flow volumes and for the volume of the highest 10 percent of daily flows. Calibration results for streamflow volumes of the lowest 50 percent of daily flows were considered poor. The daily streamflow calibration at U.S. Geological Survey streamflow-gaging station 08210000 Nueces River near Three Rivers, Tex., had the lowest (best) root mean square error, and U.S. Geological Survey streamflow-gaging station 08194500 Nueces River near Tilden, Tex., had the highest root mean square error expressed as a percentage of the mean flow rate. The mean daily reservoir volume during 1961–2008 was 182,000 acre-feet. Simulated mean daily reservoir volume was within 9 percent of this computed volume. Selected results of the model include streamflow yields for the subwatersheds and water-balance information for the Carrizo–Wilcox aquifer outcrop area. For the entire model domain, the area-weighted mean streamflow yield from 1961 to 2008 was 1.12 inches/year. The mean annual rainfall on the outcrop area during the 1961–2008 simulation period was 21.7 inches. Of this rainfall, an annual mean of 20.1 inches (about 93 percent) was simulated as evapotranspiration, 1.2 inches (about 6 percent) was simulated as groundwater recharge, and 0.5 inches (about 2 percent) was simulated as surface runoff.

Texas↗
USGS2011

Occurrence and distribution of pesticides in surface waters of the Hood River basin, Oregon, 1999-2009

The U.S. Geological Survey analyzed pesticide and trace-element concentration data from the Hood River basin collected by the Oregon Department of Environmental Quality (ODEQ) from 1999 through 2009 to determine the distribution and concentrations of pesticides in the basin's surface waters. Instream concentrations were compared to (1) national and State water-quality standards established to protect aquatic organisms and (2) concentrations that cause sublethal or lethal effects in order to assess their potential to adversely affect the health of salmonids and their prey organisms. Three salmonid species native to the basin are listed as "threatened" under the U.S. Endangered Species Act: bull trout, steelhead, and Chinook salmon. A subset of 16 sites was sampled every year by the ODEQ for pesticides, with sample collection targeted to months of peak pesticide use in orchards (March-June and September). Ten pesticides and four pesticide degradation products were analyzed from 1999 through 2008; 100 were analyzed in 2009. Nineteen pesticides were detected: 11 insecticides, 6 herbicides, and 2 fungicides. Two of four insecticide degradation products were detected. All five detected organophosphate insecticides and the one detected organochlorine insecticide were present at concentrations exceeding water-quality standards, sublethal effects thresholds, or acute toxicity values in one or more samples. The frequency of organophosphate detection in the basin decreased during the period of record; however, changes in sampling schedule and laboratory reporting limits hindered clear analysis of detection frequency trends. Detected herbicide and fungicide concentrations were less than water-quality standards, sublethal effects thresholds, or acute toxicity values. Simazine, the most frequently detected pesticide, was the only herbicide detected at concentrations within an order of magnitude (factor of 10) of concentrations that impact salmonid olfaction. Some detected pesticides are of concern, not for their toxicity alone, but for their ability to potentiate the harmful impacts of other pesticides, particularly organophosphates, on salmonids or their prey. Many samples contained mixtures of pesticides, but the effects to salmonids of relevant mixtures at environmentally realistic concentrations for the basin are unknown. Trace-element concentration data, although limited, indicate that eight trace elements are also of concern for their potential to harm salmonid health. The dataset is limited with regard to the spatial and seasonal distribution of pesticides and trace elements in all salmonid-bearing streams, the presence of particle-bound pesticides, and the presence of several unmonitored pesticides known to be used in the basin.

Oregon↗
USGS2014

Water quality and algal community dynamics of three deepwater lakes in Minnesota utilizing CE-QUAL-W2 models

Water quality, habitat, and fish in Minnesota lakes will potentially be facing substantial levels of stress in the coming decades primarily because of two stressors: (1) land-use change (urban and agricultural) and (2) climate change. Several regional and statewide lake modeling studies have identified the potential linkages between land-use and climate change on reductions in the volume of suitable lake habitat for coldwater fish populations. In recent years, water-resource scientists have been making the case for focused assessments and monitoring of sentinel systems to address how these stress agents change lakes over the long term. Currently in Minnesota, a large-scale effort called “Sustaining Lakes in a Changing Environment” is underway that includes a focus on monitoring basic watershed, water quality, habitat, and fish indicators of 24 Minnesota sentinel lakes across a gradient of ecoregions, depths, and nutrient levels. As part of this effort, the U.S. Geological Survey, in cooperation with the Minnesota Department of Natural Resources, developed predictive water quality models to assess water quality and habitat dynamics of three select deepwater lakes in Minnesota. The three lakes (Lake Carlos in Douglas County, Elk Lake in Clearwater County, and Trout Lake in Cook County) were assessed under recent (2010–11) meteorological conditions. The three selected lakes contain deep, coldwater habitats that remain viable during the summer months for coldwater fish species. Hydrodynamics and water-quality characteristics for each of the three lakes were simulated using the CE-QUAL-W2 model, which is a carbon-based, laterally averaged, two-dimensional water-quality model. The CE-QUAL-W2 models address the interaction between nutrient cycling, primary production, and trophic dynamics to predict responses in the distribution of temperature and oxygen in lakes. The CE-QUAL-W2 models for all three lakes successfully predicted water temperature, on the basis of the two metrics of absolute mean error and root mean square error, using measured inputs of water temperature and nutrients. One of the main calibration tools for CE-QUAL-W2 model development was the vertical profile temperature data, available for all three lakes. For all three lakes, the absolute mean error and root mean square error were less than 1.0 degree Celsius and 1.2 degrees Celsius, respectively, for the different depth ranges used for vertical profile comparisons. In Lake Carlos, simulated water temperatures compared better to measured water temperatures in the epilimnion than in the hypolimnion. The reverse was true for the other two lakes, Elk Lake and Trout Lake, where the simulated results were slightly better for the hypolimnion than the epilimnion. The model also was used to approximate the location of the thermocline throughout the simulation periods, approximately April to November, in all three lake models. Deviations between the simulated and measured water temperatures in the vertical lake profile commonly were because of an offset in the timing of thermocline shifts rather than the simulated results missing thermocline shifts altogether.

Minnesota↗
USGS2011

Factors that influence the hydrologic recovery of wetlands in the Northern Tampa Bay area, Florida

Reductions in groundwater withdrawals from Northern Tampa Bay well fields were initiated in mid-2002 to improve the hydrologic condition of wetlands in these areas by allowing surface and groundwater levels to recover to previously higher levels. Following these reductions, water levels at some long-term wetland monitoring sites have recovered, while others have not recovered as expected. To understand why water levels for some wetlands have not increased, nine wetlands with varying impacts from well field pumping were examined based on four factors known to influence the hydrologic condition of wetlands in west-central Florida. These factors are the level of the potentiometric surface of the Upper Floridan aquifer underlying the wetland, recent karst activity near and beneath the wetland, permeability of the underlying sediments, and the topographic position of the wetland in the landscape. The combination of two factors, the presence of recent karst activity below or near the wetlands and the depth to the potentiometric surface of the Upper Floridan aquifer below the wetlands, had the most influence on the hydrologic recovery of the study wetlands. The study wetlands are located in an area where numerous localized surface or buried depressions (karst features or sinkholes) are common throughout the mantled karst landscape, which increases the hydrologic connection between the wetlands and the underlying aquifers. Breaches or breaks in the underlying sediments or in the intermediate confining unit due to recent karst subsidence activity act as pathways for downward leakage. For the study wetlands, the leakage potential increased when the vertical separation between the potentiometric surface of the Upper Floridan aquifer and the wetland-bottom elevation (a surrogate for the wetland water level) increased. The increase in the potentiometric surface of the Upper Floridan aquifer below the wetland was the primary factor influencing the hydrologic recovery of the study wetlands, even in areas affected by karst subsidence. For one of the study wetlands influenced by karst subsidence (S-44 Cypress at Starkey well field), the potentiometric surface of the Upper Floridan aquifer increased to the level of the wetland-bottom elevation following the reductions in groundwater withdrawals. Despite the karst subsidence in the wetland, having the level of the potentiometric surface just below the wetland bottom limited the downward leakage potential and resulted in an increase in the flooded area and duration of the wetland hydroperiod. In contrast, two study wetlands affected by karst subsidence (W-12 Cypress and W-16 Marsh at Cypress Creek) remained mostly dry during the period of groundwater withdrawal reductions, even though the median elevation of the potentiometric surface of the Upper Floridan aquifer rose about 5 feet in this area of the well field. These wetlands are located in an area of the well field where large groundwater withdrawals are concentrated, and during the last 20 years (1989-2009) the wetlands were inundated only during periods of extreme rainfall. During these brief inundation periods, the wetland water levels receded after 1 to 2 months, much more rapidly than wetlands located in areas without karst subsidence or concentrated pumping, indicating the increased leakage between the wetlands and underlying aquifers. Because of this interconnection, water levels in these wetlands and others impacted by karst subsidence in this region will not recover if the potentiometric surface of the Upper Floridan aquifer remains at its current (2009) elevation (median distance of about 10 feet below the wetland-bottom elevation). Low permeability sediments and the absence of karst features underlying the wetlands had a positive influence on the wetland recovery following the reductions in groundwater withdrawals. In these settings, intact low permeability subsurface layers help maintain water within and beneath the wetland, and limit the downward leakage potential to the Upper Floridan aquifer. For wetlands in these settings, the increase in potentiometric surface of the Upper Floridan aquifer below the study wetland-bottom elevations resulted in an increase in the flooded area and the duration of the wetland hydroperiod. Although of less importance than the other three factors, a low-lying topographical position benefited the hydrologic condition of several of the study wetlands (S-68 Cypress and W-12 Cypress) both before and after the reductions in groundwater withdrawals. Compared to wetlands in a higher topographical position, those in a lower position had longer hydroperiods because of their greater ability to receive more runoff from higher elevation wetlands and to establish surface-water connections to other isolated wetlands and surface-water bodies through low-lying surface-water channels during wet conditions. In addition, wetlands in low-lying areas benefited from groundwater inflow when groundwater levels were higher than wetland water levels.

Scientific Investigations Report↗
USGS2015

Data Collection and Simulation of Ecological Habitat and Recreational Habitat in the Shenandoah River, Virginia

This report presents updates to methods, describes additional data collected, documents modeling results, and discusses implications from an updated habitat-flow model that can be used to predict ecological habitat for fish and recreational habitat for canoeing on the main stem Shenandoah River in Virginia. Given a 76-percent increase in population predictions for 2040 over 1995 records, increased water-withdrawal scenarios were evaluated to determine the effects on habitat and recreation in the Shenandoah River. Projected water demands for 2040 vary by watershed: the North Fork Shenandoah River shows a 55.9-percent increase, the South Fork Shenandoah River shows a 46.5-percent increase, and the main stem Shenandoah River shows a 52-percent increase; most localities are projected to approach the total permitted surface-water and groundwater withdrawals values by 2040, and a few localities are projected to exceed these values. The habitat model used for this study evaluates the suitability of ecological habitat, represented by fish, and recreational habitat, represented by canoeing, based on depth, velocity, and substrate conditions, which are weighted for the physical habitat types (riffles, runs, or pools) present within a stretch of river. Weighted usable-habitat area in the Lockes Mill reach was maximized for adult smallmouth bass and sub-adult smallmouth bass ( Micropterus dolomieu ) and river chub ( Nocomis micropogon ) when streamflows were equal to median flow (900 cubic feet per second) for summer months. Ecological maximum weighted usable-habitat areas for smaller fish, such as spotfin or satinfin shiner ( Cyprinella spp.), margined madtom ( Noturus insignis ), and juvenile redbreast sunfish ( Lepomis auritus ) occurred with 10 th percentile flows (482 cubic feet per second) and lower. Recreational weighted usable-habitat areas for canoeing were maximized when streamflows were above the 75 th percentile (1,410 cubic feet per second). During historic droughts, streamflows were less than the 10 th percentile, and adult smallmouth bass and sub-adult smallmouth bass habitat was below normal for the majority of days during at least 2 months of the summer. When streamflows were less than the lowest 7-day average in a 10-year period, or 7Q10 flow (357 cubic feet per second), margined madtom, river chub, and sub-adult redbreast sunfish habitat areas were below normal as well. Streamflows that limit most fish species habit availability range from 300 to 500 cubic feet per second. For the drought years simulated, flows that were equal to or less than the 10 th percentile for summer months did not provide adequate depth for canoe passage through riffle habitats. A modeling limitation for higher flows than those studied during development of the habitat-suitability criteria is that modeled habitat availability will decrease as flows increase. Time-series analyses were used to investigate changes in habitat availability with increased water withdrawals of 10, 20, and almost 50 percent (48.6 percent) up to the 2040 amounts projected by local water supply plans. Adult and sub-adult smallmouth bass frequently had habitat availability outside the normal range for habitat conditions during drought years, yet 10- or 20-percent increases in withdrawals did not contribute to a large reduction in habitat. When withdrawals were increased by 50 percent, there was an additional decrease in habitat. During 2002 drought scenarios, reduced habitat availability for sub-adult redbreast sunfish or river chub was only slightly evident with 50-percent increased withdrawal scenarios. Recreational habitat represented by canoeing decreased lower than normal during the 2002 drought. For a recent normal year, like 2012, increased water-withdrawal scenarios did not affect habitat availability for fish such as adult and sub-adult smallmouth bass, sub-adult redbreast sunfish, or river chub. Canoeing habitat availability was within the normal range most of 2012, and increased water-withdrawal scenarios showed almost no affect. For both ecological fish habitat and recreational canoeing habitat, the antecedent conditions (habitat within normal range of habitat or below normal) appear to govern whether additional water withdrawals will affect habitat availability. As human populations and water demands increase, many of the ecological or recreational stresses may be lessened by managing the timing of water withdrawals from the system.

Virginia↗
USGS2020

Updating data inputs, assessing trends, and evaluating a method to estimate probable high groundwater levels in selected areas of Massachusetts

A method to estimate the probable high groundwater level in Massachusetts, excluding Cape Cod and the islands, was developed in 1981. The method uses a groundwater measurement from a test site, groundwater measurements from an index well, and a distribution of high groundwater levels from wells in similar geologic and topographic settings. The U.S. Geological Survey, in cooperation with the Massachusetts Department of Environmental Protection, conducted an update to the Frimpter method for estimating the probable high groundwater levels in Massachusetts. The study evaluated the potential changes to the method resulting from four decades of additional groundwater-level data and the expansion of the network of wells for monitoring groundwater levels. The differences and potential benefits of daily, as opposed to monthly, measurements in the application of the method were examined because of the increased availability of high-frequency (subdaily) groundwater-level data. The study also considered long-term trends in groundwater levels that may alter the accuracy of the method. Finally, the accuracy of the estimated high groundwater levels was evaluated, and improved implementation guidance was prepared. For this study, groundwater levels in 153 wells in Massachusetts and surrounding States with records with lengths of 16 to 78 years were analyzed. The highest recorded groundwater levels ranged from 1.2 feet (ft) above land surface (flooded conditions) to 45.8 ft below land surface, with a median of 4.6 ft below land surface. The maximum annual groundwater-level range was 1.4 to 17.9 ft, with a median of 5.5 ft. The within-month variation, maximum annual groundwater-level range, and highest recorded groundwater level were computed using daily mean groundwater-level values from 28 wells with continuous records. The use of daily data resulted in larger maximum annual groundwater-level ranges (0.02 to 2.94 ft larger, with a median of 0.58 ft larger) and shallower highest-recorded groundwater levels (0.0 to 1.60 ft shallower, with a median of 0.18 ft shallower) than computations based on monthly measurements in the same wells. Statistical tests showed moderate to strong evidence of trends in measurements of both high and low groundwater levels within most of the periods during which water levels were analyzed. High groundwater levels rose beneath the land surface at most sites during four of the six periods used for analysis (1966–2015, 1986–2015, 1991–2010, and 1981–2010). Low groundwater levels also increased at many sites during most of the periods evaluated, but this trend was less widespread than the similar trends in high groundwater levels, and the trend was to deeper low groundwater levels at more sites than the trend to deeper high groundwater levels. There was no clear trend in annual groundwater-level ranges at most sites during the six periods analyzed. In general, the Frimpter method predicted shallower (higher) high groundwater levels than were observed but correctly classified sites according to their suitabilities for unmounded septic systems. The mean error of the predictions (difference between the estimated and observed groundwater levels) ranged from −3.23 ft to −1.40 ft for various approaches to estimating the groundwater-level range and selecting an index well. The method correctly classified 83 to 86 percent of monitoring-well sites according to their suitability for an unmounded septic system for many approaches to estimating the annual groundwater-level range and selecting an index well. The approach selected for estimating the annual groundwater-level range and selecting an index well will depend upon the importance of an accurate estimate of the high groundwater level as compared to the importance of an estimated high groundwater level that is less likely to be exceeded.

Connecticut, Massachusetts, New Hampshire, Rhode I↗
USGS2007

Use of borehole-radar methods to monitor a steam-enhanced remediation pilot study at a quarry at the former Loring Air Force Base, Maine

Single-hole radar reflection and crosshole radar tomography surveys were used in conjunction with conventional borehole-geophysical methods to evaluate the effectiveness of borehole-radar methods for monitoring the movement of steam and heat through fractured bedrock. The U.S. Geological Survey, in cooperation with U.S. Environmental Protection Agency (USEPA), conducted surveys in an abandoned limestone quarry at the former Loring Air Force Base during a field-scale, steam-enhanced remediation (SER) pilot project conducted by the USEPA, the U.S. Air Force, and the Maine Department of Environmental Protection to study the viability of SER to remediate non-aqueous phase liquid contamination in fractured bedrock. Numerical modeling and field experiments indicate that borehole-radar methods have the potential to monitor the presence of steam and to measure large temperature changes in the limestone matrix during SER operations. Based on modeling results, the replacement of water by steam in fractures should produce a decrease in radar reflectivity (amplitude of the reflected wave) by a factor of 10 and a change in reflection polarity. In addition, heating the limestone matrix should increase the bulk electrical conductivity and decrease the bulk dielectric permittivity. These changes result in an increase in radar attenuation and an increase in radar-wave propagation velocity, respectively. Single-hole radar reflection and crosshole radar tomography data were collected in two boreholes using 100-megahertz antennas before the start of steam injection, about 10 days after the steam injection began, and 2 months later, near the end of the injection. Fluid temperature logs show that the temperature of the fluid in the boreholes increased by 10?C (degrees Celsius) in one borehole and 40?C in the other; maximum temperatures were measured near the bottom of the boreholes. The results of the numerical modeling were used to interpret the borehole-radar data. Analyses of the single-hole radar reflection data showed almost no indication that steam replaced water in fractures near the boreholes because (1) no change of polarity was observed in the radar reflections; (2) variations in the measured traveltimes were unsubstantial; and (3) most of the observed decreases in reflectivity were too small to have resulted from the replacement of water by steam. Analyses of the crosshole radar tomography data also support the conclusion that steam did not replace water in the fractures around the boreholes because traveltime-difference and attenuation-difference tomograms showed only small decreases in velocity and small increases in attenuation accompanying the steam injection. The radar data are consistent with an increase in the conductivity of the limestone as a result of heating of the limestone matrix near the boreholes. Single-hole radar reflection data collected near the end of the steam injection near the bottom of the borehole with the largest temperature increase showed substantial attenuation. Also, reflector analysis showed small decreases in the amplitudes of radar-wave reflections in data collected before injection and data collected near the end of the collection period. In the crosshole radar tomography data, decreases in velocity and small increases in attenuation also are consistent with temperature increases in the matrix.

Maine↗
USGS2021

Estimation of dissolved-solids concentrations using continuous water-quality monitoring and regression models at four sites in the Yuma area, Arizona and California, January 2017 through March 2019

Multiple linear regression models were developed to estimate dissolved-solids concentrations in water at four sites in the Yuma area between Imperial Dam, Arizona and California and the southerly international boundary with Mexico at San Luis, Arizona. Continuous and discrete water-quality data were collected at gaging stations in the Colorado River upstream from Imperial Dam, Arizona-California, the Colorado River below Cooper wasteway near Yuma, Arizona, the Yuma Main Drain above Arizona–Sonora, Mexico boundary, and the 242 lateral above Main Drain at the Arizona–Sonora boundary. Continuous specific conductance and water temperature data were collected at each site between January 2017 and March 2019. Bi-weekly to monthly dissolved-solids water samples were collected during the same period. Continuous specific conductance data collected at the Colorado River below Cooper wasteway were affected by poorly mixed streamflow during periods when the Pilot Knob Hydro-electric Plant was releasing water to the river. The continuous specific conductance data for the site downstream from Cooper wasteway were corrected using mean specific conductance values computed from cross-section measurements collected during site visits. Continuous specific conductance data were affected by sensor fouling issues at the 242 lateral site, and continued operation at the site would require more frequent visits for cleaning and service to ensure data quality. During the study, instream specific conductance readings ranged from 966 to 3,030 microsiemens per centimeter (μS/cm) at 25 degrees Celsius. Computed dissolved-solids concentrations from discrete samples ranged from 690 to 2,580 milligrams per liter (mg/L). Dissolved-solids concentrations were estimated from regression models using the optimal relation between dissolved solids and environmental factors, such as specific conductance, water temperature, dissolved oxygen, streamflow, and seasonality. Specific conductance was the primary factor at all four sites and explained 87.6 to 94 percent of variation in dissolved solids. Water temperature, as an indicator of seasonality, was determined to be a statistically significant secondary factor at both the Colorado River above Imperial Dam and Colorado River below Cooper wasteway sites explaining an additional 6.9 and 2.1 percent of variation in dissolved solids, respectively. Regression models explained 87.6 to 96.9 percent of the variation in dissolved solids; the root mean square error in the modeled data ranged between about 6 and 27 mg/L.

Arizona, California↗
USGS2005

Water-quality, phytoplankton, and trophic-status characteristics of Big Base and Little Base lakes, Little Rock Air Force Base, Arkansas, 2003-2004

Little Rock Air Force Base is the largest C-130 base in the Air Force and is the only C-130 training base in the Department of Defense. Little Rock Air Force Base is located in central Arkansas near the eastern edge of the Ouachita Mountains, near the Mississippi Alluvial Plain, and within the Arkansas Valley Ecoregion. Habitats include upland pine forests, upland deciduous forest, broad-leaved deciduous swamps, and two small freshwater lakes?Big Base Lake and Little Base Lake. Big Base and Little Base Lakes are used primarily for recreational fishing by base personnel and the civilian public. Under normal (rainfall) conditions, Big Base Lake has a surface area of approximately 39 acres while surface area of Little Base Lake is approximately 1 acre. Little Rock Air Force Base personnel are responsible for managing the fishery in these two lakes and since 1999 have started a nutrient enhancement program that involves sporadically adding fertilizer to Big Base Lake. As a means of determining the relations between water quality and primary production, Little Rock Air Force Base personnel have a need for biological (phytoplankton density), chemical (dissolved-oxygen and nutrient concentrations), and physical (water temperature and light transparency) data. To address these monitoring needs, the U.S. Geological Survey in cooperation with Little Rock Air Force Base, conducted a study to collect and analyze biological, chemical, and physical data. The U.S. Geological Survey sampled water quality in Big Base Lake and Little Base Lake on nine occasions from July 2003 through June 2004. Because of the difference in size, two sampling sites were established on Big Base Lake, while only one site was established on Little Base Lake. Lake profile data for Big Base Lake indicate that low dissolved- oxygen concentrations in the hypolimnion probably constrain most fish species to the upper 5-6 feet of depth during the summer stratification period. Dissolved-oxygen concentrations in Big Base Lake below a depth of 6 feet generally were less than 3 milligrams per liter for summer months that were sampled in 2003 and 2004. Some evidence indicates that phosphorus was limiting primary production during the sampling period. Dissolved nitrogen constituents frequently were detected in water samples (indicating availability) but dissolved phosphorus constituents-orthophosphorus and dissolved phosphorus-were not detected in any samples collected at the two lakes. The absence of dissolved phosphorus constituents and presence of total phosphorus indicates that all phosphorus was bound to suspended material (sediment particles and living organisms). Nitrogen:phosphorus ratios on most sampling occasions tended to be slightly higher than 16:1, which can be interpreted as further indication that phosphorus could be limiting primary production to some extent. An alkalinity of 20 milligrams per liter of calcium carbonate or higher is recommended to optimize nutrient availability and buffering capacity in recreational fishing lakes and ponds. Median values for water samples collected at the three sites ranged from 12-13 milligrams per liter of calcium carbonate. Alkalinities ranged from 9-60 milligrams per liter of calcium carbonate, but 13 of 17 samples collected at the deepest site had alkalinities less than 20 milligrams per liter of calcium carbonate. Results of three trophic-state indices, and a general trophic classification, as well as abundant green algae and large growths of blue-green algae indicate that Big Base Lake may be eutrophic. Trophic-state index values calculated using total phosphorus, chlorophyll a, and Secchi disc measurements from both lakes generally exceeded criteria at which lakes are considered to be eutrophic. A second method of determining lake trophic status-the general trophic classification-categorized the three sampling sites as mesotrophic or eutrophic. Green algae were found to be in abundance throughout mos

Scientific Investigations Report↗
USGS2023

Assessment of prerestoration water quality in the Herring River to support adaptive management at the Cape Cod National Seashore

In 2020 and 2021, the U.S. Geological Survey, Cape Cod National Seashore of the National Park Service, and Friends of Herring River cooperated to assess nutrient and suspended sediment concentrations across the ocean-estuary boundary at a dike on the Herring River on Chequessett Neck Road in Wellfleet, Massachusetts, that has restricted saltwater inputs by regulating water inflow through three culverts or sluiceways into the watershed for more than 100 years. The dike is slated to be removed, and the purpose of this project was to characterize natural variability of nutrient and suspended sediment concentration during flood tide and ebb tide conditions at the dike based on seasonal and environmental variables. This baseline can be used to assess if removal of the dike is likely to result in measurable changes in water quality. Data from the current [2023] study were aggregated with previously published data from November 2015 through September 2018 to provide a long-term record. Samples for the current [2023] study were collected from flood and following ebb tides approximately twice per month from June 2020 through December 2021 at fixed time intervals after the beginning of the tides. Samples were analyzed for nitrate plus nitrite, ammonium, total dissolved nitrogen, total nitrogen, orthophosphate, total dissolved phosphorus, total phosphorus, silica, dissolved organic carbon, and suspended sediment. Constituent concentrations generally were lower using fixed time sampling than in previous studies that used flow-weighted composite sampling, except for nitrate plus nitrite and orthophosphate. Concentrations of nitrate plus nitrite, ammonium, total nitrogen, total dissolved nitrogen, silica, and dissolved organic nitrogen generally were higher on the ebb tide than on the flood tide. By contrast, concentrations of orthophosphate, total phosphorus, and total dissolved phosphorus were generally similar between flood and ebb tides. Most nutrient concentrations except silica and ammonium varied seasonally on flood and ebb tides. Phosphorus species, total nitrogen, and dissolved organic carbon concentrations generally peaked in mid- to late summer and were lowest in winter. For nitrate, the reverse was true. Nutrient concentrations generally were higher on the ebb tide than on the flood tide except for total dissolved phosphorus and total phosphorus where differences between flood and ebb tide depended on season. Constituent concentrations were similar between spring, neap, and midamplitude tides on both the flood and ebb tides. Nitrate, ammonium, total nitrogen, and silica concentrations were positively correlated with precipitation and runoff. Orthophosphate, total dissolved phosphorus, total phosphorus, and dissolved organic carbon were positively correlated with surface air temperature, downwelling shortwave radiation, and ocean water temperature. Nitrate, ammonium, and silica concentration were negatively correlated with surface air temperature and ocean water temperature. Orthophosphate and total dissolved phosphorus were negatively correlated with runoff. Nitrate plus nitrite, ammonium, and silica concentrations were negatively correlated with downwelling shortwave radiation.

Massachusetts↗
USGS2004

Sources of phosphorus to the Carson River upstream from Lahontan Reservoir, Nevada and California, Water Years 2001-02

Discharge of treated municipal-sewage effluent to the Carson River in western Nevada and eastern California ceased by 1987 and resulted in a substantial decrease in phosphorus concentrations in the Carson River. Nonetheless, concentrations of total phosphorus and suspended sediment still commonly exceed beneficial-use criteria established for the Carson River by the Nevada Division of Environmental Protection. Potential sources of phosphorus in the study area include natural inputs from undisturbed soils, erosion of soils and streambanks, construction of low-head dams and their destruction during floods, manure production and grazing by cattle along streambanks, drainage from fields irrigated with streamwater and treated municipal-sewage effluent, ground-water seepage, and urban runoff including inputs from golf courses. In 2000, the U.S. Geological Survey (USGS), in cooperation with Carson Water Subconservancy District, began an investigation with the overall purpose of providing managers and regulators with information necessary to develop and implement total maximum daily loads for the Carson River. Two specific goals of the investigation were (1) to identify those reaches of the Carson River upstream from Lahontan Reservoir where the greatest increases in phosphorus and suspended-sediment concentrations and loading occur, and (2) to identify the most important sources of phosphorus within the reaches of the Carson River where the greatest increases in concentration and loading occur. Total-phosphorus concentrations in surface-water samples collected by USGS in the study area during water years 2001-02 ranged from <0.01 to 1.78 mg/L and dissolved-orthophosphate concentrations ranged from <0.01 to 1.81 mg/L as phosphorus. In streamflow entering Carson Valley from headwater areas in the East Fork Carson River, the majority of samples exceeding the total phosphorus water-quality standard of 0.1 mg/L occur during spring runoff (March, April, and May) when suspended-sediment concentrations are high. Downstream from Carson Valley, almost all samples exceed the water-quality standard, with the greatest concentrations observed during spring and summer months. Estimated annual total-phosphorus loads ranged from 1.33 tons at the West Fork Carson River at Woodfords to 43.41 tons at the Carson River near Carson City during water years 2001-02. Loads are greatest during spring runoff, followed by fall and winter, and least during the summer, which corresponds to the amount of streamflow in the Carson River. The estimated average annual phosphorus load entering Carson Valley was 21.9 tons; whereas, the estimated average annual phosphorus load leaving Carson Valley was 37.8 tons, for an annual gain in load across Carson Valley of 15.9 tons. Thus, about 58 percent of the total-phosphorus load leaving Carson Valley on an annual basis could be attributed to headwater reaches upstream from Carson Valley. During spring and summer (April 1-September 30) an average of 85 percent of the total-phosphorus load leaving Carson Valley could be attributed to headwater reaches. During fall and winter (October 1-March 31) only 17 percent of the phosphorus load leaving Carson Valley could be attributed to headwater reaches. The composition of the phosphorus changes during summer from particulate phosphorus entering Carson Valley to dissolved orthophosphate leaving Carson Valley. Particulate phosphorus entering Carson Valley could be settling out when water is applied to fields and be replaced by dissolved orthophosphate from other sources. Alternatively, the particulate phosphorus could be converted to dissolved orthophosphate as it travels across Carson Valley. Data collected during the study are not sufficient to distinguish between the two possibilities. Eagle Valley and Dayton-Churchill Valleys may act as sinks for phosphorus. On an annual basis, during water years 2001-02, about 90 percent of the phosphorus entering Eagle Valley left the

Scientific Investigations Report↗
USGS2022

Seasonal and long-term clarity trend assessment of Lake Tahoe, California–Nevada

The clarity of Lake Tahoe, observed using a Secchi disk on a regular basis since the late 1960s, continues to be a sentinel metric of lake health. Water clarity is influenced by physical and biological processes and has declined in the five decades of monitoring, revealing differences between summer (June–September) and winter (December–March). This document summarizes key findings of a study of Lake Tahoe water clarity, including long-term variability and the relative importance of several influencing variables and processes. This study, prepared in cooperation with the Nevada Division of Environmental Protection, focused on (1) an apparent divergence in clarity trends between summer and winter periods, (2) observed changes in in-lake physical and ecological variables that may influence or control seasonal and annual clarity trends, and (3) five research hypotheses regarding lake clarity that were developed by Lake Tahoe management agencies. Previously collected data were used to complete this study. Trend analysis confirmed that winter clarity stabilized (that is, there is no longer a statistically significant trend up or down) during the last 20 years. Evaluation of clarity for selected months in the 50-year Secchi disk clarity dataset showed that only two summer months, July and August, had statistically significant decreases in clarity from 2000–19. Different subsets of available data were analyzed to reveal the presence or absences of trends for each season, decade, and month. Five hypotheses related to lake clarity were part of the study described by this report. Hypothesis 1 stated that clarity is controlled predominantly by the distribution and volumetric density of fine particles in suspension. This hypothesis was studied using available data describing in-lake fine (0–20 micrometers) particles from 2008–19. Water clarity was negatively correlated with in-lake particle abundance, with particles in the 1.0-4.6 μm range having the greatest effect, consistent with light-scattering theory. Estimated abundances of diatoms of the genus Cyclotella also were found to be negatively correlated with clarity. Data limitations precluded a complete investigation of hypothesis 2, which stated that the observed improvements in winter water clarity are a response to decreasing fine suspended-sediment concentrations in the lake resulting from load reductions from upland sources in and near urbanized areas. Data describing fine-sediment loading from urban areas to the lake were only available since 2014, and only once or twice per month. A slight, statistically significant, negative correlation was identified between urban fine-particle loading and monthly lake clarity with a 4-month lag. Particle abundance in monitored streams is highly correlated with simultaneous particle abundance in the lake. Hypothesis 3 stated that changing hydrodynamic conditions in the lake are increasing thermal stability and resistance to mixing. Trend analyses performed on stability index and buoyancy frequency time series computed from long-term observations of lake temperatures support the hypothesis that hydrodynamic conditions have evolved since 1969 to increase the lake’s resistance to mixing. The date of maximum mixing in winter has become progressively earlier in the year. Lake density stratification, defined using the stability index, is commencing earlier in the year and extending a month longer than in the early years of the monitoring program. Hypothesis 4 stated that the trend of decreasing summer clarity is a result of earlier, prolonged, and more intense stratification. Statistically significant correlations were found between summer clarity and (1) date of onset of stratification, (2) duration of stratification, and (3) buoyancy frequency. Hypothesis 5 stated that ecological (food web) interactions are causing changes in the trends of seasonal or annual clarity; data supporting hypothesis 5 were limited to examples from other systems and to intermittent monitoring of Lake Tahoe and Emerald Bay. The resulting narrative assessment was motivated by a 6-year study of Mysis shrimp disappearance and return in Emerald Bay. The available data and a large body of published literature are consistent with the inference that Mysis shrimp-induced food web changes are causing changes in the trends of seasonal or annual clarity. This food-web study focused on the relations between introduced Mysis shrimp, the native cladocerans ( Daphnia and Bosmina ) that were largely eliminated following Mysis introduction, and the effect on fine particles within the lake. The records of Mysis and other zooplankton data for Lake Tahoe are episodic and have large gaps. Consequently, statistical analyses could not be conducted to compare zooplankton data with other variables. The long-term record, however, indicates that the key effect was a change to the phytoplankton assemblage, where larger diatoms disappeared, likely due to Mysis grazing, only to be replaced by Cyclotella that are an order-of-magnitude smaller and have increased the abundance and volumetric density of total fine particles in suspension (biotic and abiotic).

California↗