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At least 469 records · Page 26Linked to original sources
USGS1999

Selected nutrients and pesticides in streams of the eastern Iowa basins, 1970-95

Water-quality data from 17 surface-water monitoring sites were compiled for 1970 through 1995 and analyzed to determine historical waterquality conditions and possible trends in the Eastern Iowa Basins study unit as part of the U.S. Geological Survey's National Water-Quality Assessment Program. The Eastern Iowa Basins encompasses the Wapsipinicon, Cedar, Iowa, and Skunk River Basins and covers about 19,500 square miles. Seven of the monitoring sites were sampled by the Iowa Department of Natural Resources, three sites by the Minnesota Pollution Control Agency, three sites by the University of Iowa Institute for Hydraulic Research, and four sites by the U.S. Geological Survey. Water-quality analyses typically consisted of nitrate, ammonia, total nitrogen, and total phosphorus, with limited analyses available for organic nitrogen, dissolved phosphorus, dissolved orthophosphate, and water-soluble pesticides. Long-term historical nutrient and pesticide data were not available for the Wapsipinicon River Basin. Median concentrations for total nitrogen ranged from 4.6 to 9.4 milligrams per liter, and maximum concentrations of total nitrogen ranged from 4.6 to 31 milligrams per liter. The majority of nitrogen transported in surface waters of the Eastern Iowa Basins study unit is in the form of nitrate (nitrogen). Median concentrations of total phosphorus ranged from less than 0.10 to 0.66 milligram per liter, and maximum concentrations of total phosphorus ranged from less than 0.10 to 5.4 milligrams per liter. Nitrate varied seasonally. Median concentrations of nitrate were largest during the spring and the winter (6.0 to 7.0 milligrams per liter) compared to the summer and fall (2.0 to 4.0 milligrams per liter). Concentrations of nitrate greater than 10 milligrams per liter typically occurred during spring runoff. Median ammonia concentrations generally were highest during the winter (approximately 0.3-0.5 milligram per liter) compared to the spring and summer when ammonia concentrations were often close to the detection limit (0.01 milligram per liter). In general, the median concentrations of total phosphorus varied less than 0.1 milligram per liter between seasons. The statistical analysis of the nutrient data typically indicated a strong positive correlation of nitrate with streamflow. Total phosphorus concentrations with streamflow showed greater variability than nitrate, perhaps reflecting the greater potential of transport of phosphorus on sediment rather than in the dissolved phase as with nitrate. Ammonia and ammonia plus organic nitrogen showed no correlation with streamflow or a weak positive correlation. Seasonal variations and the relations of nutrients and pesticides to streamflow generally corresponded with nonpoint‑source loadings, although possible point sources for nutrients were indicated by the data at selected monitoring sites. Statistical trend tests for concentrations and loads were computed for nitrate, ammonia, and total phosphorus. Trend analysis indicated decreases for ammonia and total phosphorus concentrations at several sites and increases for nitrate concentrations at other sites in the study unit.

Iowa, Minnesota↗
USGS2022

Multiproxy paleolimnological records provide evidence for a shift to a new ecosystem state in the Northern Great Plains, USA

Wetlands in the Prairie Pothole Region of the North American Northern Great Plains perform multiple ecosystem services and are biodiversity hotspots. However, climatological changes can result in sudden shifts in these important ecosystems. For example, marked increases in precipitation in the last few decades have resulted in a widespread shift in wetlands across the Prairie Pothole Region to a new ecohydrological state. We used multiproxy analyses (diatom community composition and invertebrate stable isotopes) of 210 Pb-dated sediment cores from two adjacent, but morphologically and hydrologically different, prairie-pothole wetlands to assess the effects of hydroclimatic variability on these wetland ecosystems. Our results provide evidence that the recent ecohydrological shift in the region's wetlands is unprecedented over the past ca. 178 yr. Oxygen stable isotopes in chironomid head capsules provide a record of paleohydrology changes. The most recent sediments (i.e., those deposited after the state shift) from both wetlands revealed novel changes in diatom communities that differed greatly from earlier community compositions. In addition, a depleted signal in deuterium and 13 C carbon stable isotopes observed in chironomid head capsules and Daphnia ephippia, respectively, after 1993 is likely related to an increase in methane production in these wetlands. Our study highlights the importance of considering basin morphometry including whether a wetland has an overflow point, and multiple biological indicators to study climate-change influences on freshwater ecosystems. Research using these techniques can lead to an improved understanding of recent ecosystem shifts, an understanding that will be essential for future climate-change adaptation and mitigation in this ecologically important region.

North Dakota↗
USGS2004

Spatial databases of the Humboldt Basin mineral resource assessment, northern Nevada

This report describes the origin, generation, and format of tract map databases for deposit types that accompany the metallic mineral resource assessment for the Humboldt River Basin, northern Nevada, (Wallace and others, 2004, Chapter 2). The deposit types include pluton-related polymetallic, sedimentary rock-hosted Au-Ag, and epithermal Au-Ag. The tract maps constitute only part of the assessment, which also includes new research and data for northern Nevada, discussions on land classification, and interpretation of the assessment maps. The purpose of the assessment was to identify areas that may have a greater favorability for undiscovered metallic mineral deposits, provide analysis of the mineral-resource favorability, and present the assessment of the Humboldt River basin and adjacent areas in a digital format using a Geographic Information System (GIS).

Open-File Report↗
USGS2018

Stream fish colonization but not persistence varies regionally across a large North American river basin

Many species have distributions that span distinctly different physiographic regions, and effective conservation of such taxa will require a full accounting of all factors that potentially influence populations. Ecologists recognize effects of physiographic differences in topography, geology and climate on local habitat configurations, and thus the relevance of landscape heterogeneity to species distributions and abundances. However, research is lacking that examines how physiography affects the processes underlying metapopulation dynamics. We used data describing occupancy dynamics of stream fishes to evaluate evidence that physiography influences rates at which individual taxa persist in or colonize stream reaches under different flow conditions. Using periodic survey data from a stream fish assemblage in a large river basin that encompasses multiple physiographic regions, we fit multi-species dynamic occupancy models. Our modeling results suggested that stream fish colonization but not persistence was strongly governed by physiography, with estimated colonization rates considerably higher in Coastal Plain streams than in Piedmont and Blue Ridge systems. Like colonization, persistence was positively related to an index of stream flow magnitude, but the relationship between flow and persistence did not depend on physiography. Understanding the relative importance of colonization and persistence, and how one or both processes may change across the landscape, is critical information for the conservation of broadly distributed taxa, and conservation strategies explicitly accounting for spatial variation in these processes are likely to be more successful for such taxa.

Alabama, Florida, Georgia↗
USGS2009

Water balance dynamics in the Nile Basin

Understanding the temporal and spatial dynamics of key water balance components of the Nile River will provide important information for the management of its water resources. This study used satellite-derived rainfall and other key weather variables derived from the Global Data Assimilation System to estimate and map the distribution of rainfall, actual evapotranspiration (ETa), and runoff. Daily water balance components were modelled in a grid-cell environment at 0·1 degree (∼10 km) spatial resolution for 7 years from 2001 through 2007. Annual maps of the key water balance components and derived variables such as runoff and ETa as a percent of rainfall were produced. Generally, the spatial patterns of rainfall and ETa indicate high values in the upstream watersheds (Uganda, southern Sudan, and southwestern Ethiopia) and low values in the downstream watersheds. However, runoff as a percent of rainfall is much higher in the Ethiopian highlands around the Blue Nile subwatershed. The analysis also showed the possible impact of land degradation in the Ethiopian highlands in reducing ETa magnitudes despite the availability of sufficient rainfall. Although the model estimates require field validation for the different subwatersheds, the runoff volume estimate for the Blue Nile subwatershed is within 7·0% of a figure reported from an earlier study. Further research is required for a thorough validation of the results and their integration with ecohydrologic models for better management of water and land resources in the various Nile Basin ecosystems.

Hydrological Processes↗
USGS2018

Findings and lessons learned from the assessment of the Mexico-United States transboundary San Pedro and Santa Cruz aquifers: The utility of social science in applied hydrologic research

Study Region This study region encompasses the Transboundary San Pedro and Santa Cruz aquifers which are shared between the states of Sonora (Mexico) and Arizona (US). Special regional considerations include a semi-arid climate, basin-fill aquifers with predominantly montane recharge areas, economic drivers in the mining, trade, and military sectors, groundwater-dependent cities with expanding cones of depression, interbasin groundwater transfers, ground- and surface-water contamination, and protected aquatic and riparian habitats that act as significant migration corridors for hundreds of species, including some that are threatened and endangered. Study Focus We focus on lessons learned from the hydrologic assessment of the Transboundary San Pedro and Santa Cruz aquifers. We conducted the work, in two phases: (1) laying the groundwork and (2) implementation. The “laying the groundwork” phase consisted of binational meetings with stakeholders and key actors (agencies and individuals), and the development of an understanding of the physical, institutional, historical, and socio-political context. This led to signing of the binational Transboundary Aquifer Assessment Program (TAAP) agreement in 2009 and detailed the process for cooperation and coordination in the assessment of shared aquifers. The implementation phase began with an agreement to proceed with the study of four “focus” aquifers (Santa Cruz, San Pedro, Mesilla (Conejos-Médanos in Mexico), and Hueco Bolson (Bolsón del Hueco in Mexico)) and development of associated technical teams. Though we do include a brief discussion of the lessons learned from the physical science portion of the study, the results have been described and published elsewhere. The bulk of the paper instead focuses on the findings and lessons learned from the integration of social-science perspectives into a largely physical-science based program, since there is a growing recognition of the need for this type of approach especially in the management and assessment of transboundary aquifers. New Hydrological Insights for the Region The Sonora-Arizona effort succeeded because both countries were adequately represented, and because of flexibility of skills and ability of teams comprising both university and government scientists. Teams included social and earth scientists. Including the social sciences was critical to research design and implementation, and to addressing the cultural, institutional, and socio-political contexts of transboundary aquifer assessment. Significant components of the continuing implementation phase include strategic planning, data compilation and analysis, cross-border integration of datasets, geophysical and geochemical surveys, and internal, peer, and stakeholder engagement.

Arizona, Sonora↗
USGS2026

Network-wide assessment of soil water content calibration and sensitivity to biomass proxies using cosmic-ray neutron sensing in the Roaring Fork Basin, Colorado

Soil water content (SWC) is a key state variable of the climate system but is often uncertain in water balance monitoring, especially in alpine environments. SWC measurements can be challenging in alpine environments due to the topography and rocky soils. In 2022, the US Geological Survey's Next Generation Water Observing System Program began research to evaluate water balance monitoring technologies, including cosmic-ray neutron sensors (CRNS). This work evaluated the uncertainty resulting from network-wide calibration of CRNS for SWC monitoring in an alpine watershed and investigated the stability of the calibration parameters across space and time, focusing on potential influence of biomass dynamics. Fifteen stations with moderated and unmoderated (bare) CRNS were deployed and made operational within the Roaring Fork Basin in west-central Colorado. The root mean squared error of the network-wide calibration using the moderated CRNS was 0.042 or 0.047 cm 3 cm −3 , depending on the calibration equation used. Relative SWC dynamics from CRNS were correlated with the in situ probes with a correlation coefficient of 0.91 or 0.87 (depending on calibration equation). We did not find significant relationships between the calibration parameters and stationary site-specific variables. However, the calibration parameters derived from in situ probe SWC dynamics varied over time and were correlated with biomass proxies of cumulative growing degree-day, cumulative growing season index, and bare neutron counts. Future use of the CRNS network can leverage the reliable relative SWC data from network-wide calibration for watershed modeling and continue to research sensitivity of bare neutron measurements to biomass dynamics.

Colorado↗
USGS2023

Backpack satellite transmitters reduce survival but not nesting propensity or success of greater sage-grouse

Telemetry technology is ubiquitous for studying the behavior and demography of wildlife, including the use of traditional very high frequency (VHF) radio telemetry and more recent methods that record animal locations using global positioning systems (GPS). Satellite-based GPS telemetry allows researchers to collect high spatial–temporal resolution data remotely but may also come with additional costs. For example, recent studies from the southern Great Basin suggested GPS transmitters attached via backpacks may reduce the survival of greater sage-grouse ( Centrocercus urophasianus ) relative to VHF transmitters attached via collars that have been in use for decades. While some evidence suggests GPS backpacks reduce survival, no studies have examined the effects of GPS backpacks on breeding behavior and success. Therefore, we compared survival, breeding behavior, and nest success of sage-grouse hens marked with both VHF collars and GPS backpack transmitter over a 7-year period in central Idaho, USA. GPS backpacks reduced spring–summer survival of sage-grouse hens relative to hens with VHF collars, where daily mortality probability was 68%–82% higher from March 1 to August 1. Yet satellite GPS backpacks did not consistently affect nest success or the likelihood or timing of nest initiation relative to VHF collars. Daily nest survival varied annually and with timing of nest initiation and nest age, but marginal effects of transmitter type were statistically insignificant and interactions between transmitter type and study year produced no meaningful patterns. Our results corroborate recent studies for the effect of satellite GPS backpacks on sage-grouse survival, but also suggest that these transmitters do not appear to affect components of fecundity. Our results therefore add important context to recent debate surrounding the effects of GPS backpacks on sage-grouse, and the relative strengths and weaknesses of different transmitter types for understanding behavior and population dynamics.

Idaho↗
USGS2013

Documenting utility of paddlefish otoliths for quantification of metals using inductively coupled plasma mass spectrometry

RATIONALE The otoliths of the inner ear of fishes record the environment of their surrounding water throughout their life. For paddlefish ( Polyodon spathula ), otoliths have not been routinely used by scientists since their detriments were outlined in the early 1940s. We sought to determine if paddlefish otoliths were useful for resolving elemental information contained within. METHODS Adult paddlefish were collected from two wild, self-sustaining populations in Oklahoma reservoirs in the Arkansas River basin. Juveniles were obtained from a hatchery in the Red River basin of Oklahoma. Otoliths were removed and laser ablation, inductively coupled plasma mass spectrometry (ICP-MS) was used to quantify eight elements (Li, Mg, Mn, Rb, Sr, Y, Ba, and Pb) along the core and edge portions, which were analyzed for differences between otolith regions and among paddlefish sources. RESULTS Differences were found among samples for six of the eight elements examined. Otoliths from Red River basin paddlefish born in a hatchery had significantly lower amounts of Mg and Mn, but higher levels of Rb than otoliths from wild paddlefish in the Arkansas River basin. Concentrations of Y, Sr, and Ba were reduced on the edges of adult paddlefish from both reservoirs compared with the cores. CONCLUSIONS This research shows the utility of using an ICP-MS analysis of paddlefish otoliths. Future research that seeks to determine sources of paddlefish production, such as which reservoir tributaries are most important for reproduction or what proportion of the population is composed of wild versus hatchery-produced individuals, appears promising. Published in 2013. This article is a U.S. Government work and is in the public domain in the USA.

Rapid Communications in Mass Spectrometry↗
USGS2023

No evidence for cottonwood forest decline along a flow-augmented western U.S. river

In contrast to many other arid region rivers, streamflow in the South Platte River is heavily augmented by trans-basin water imports and irrigation return flows. Hydrological changes began in the 1880s, resulting in channel narrowing and the development of a continuous Populus-Salix forest by the mid-twentieth century. We assessed the composition, structure and regeneration status of the riparian forest and identified environmental variables affecting annual Populus deltoides tree growth. We sampled forest structure at four sites in 2015, and conducted dendroecological analysis at seven additional sites in 2019. The riparian forest was dominated by P. deltoides , which occurred at all sites, comprising 79% of total tree basal area and 62% of total tree density. Age structure data indicated ongoing though episodic recruitment of P. deltoides , at least over the past ~130 years. We tested 14 linear mixed effects models to describe the effect of climate and streamflow on individual tree growth (modeled as the log of BAI, n = 237 trees). The most parsimonious model selected with AICc explained 28.6% of BAI variability, and included hydrology and climate factors during the growing season (i.e., June–August streamflow, June–July PDSI), some aspects of off-season (i.e., previous November and March) streamflow, along with tree age and study site effects. The riparian forest developed in response to, and has been maintained by, current climate conditions and water management regimes. It may be negatively affected by future climate change and increased urban water demand in the basin.

Colorado↗
USGS2025

Population structure and movement dynamics of Redband Trout in the Kootenai River basin

Redband Trout Oncorhynchus mykiss gairdneri is a species of high conservation concern in the Kootenai River basin, United States and Canada. However, identifying the specific mechanisms influencing Redband Trout is difficult as its life history structure is largely unknown. Using otolith microchemistry analysis, we found that 18.5% ( n = 66) of the fish sampled ( n = 329) were a product of mainstem Kootenai River spawning. The remainder were fish ( n = 264) that originated from one of 31 tributaries throughout the basin. Redband Trout were captured between 0 and 153 km from their assigned natal tributaries. Most migratory fish originated from tributaries in the uppermost portion of the study area and had migratory mothers. However, a diversity of life history types (e.g., multi-year resident, migratory) was observed in the population. Redband Trout were estimated to outmigrate from natal habitats at a variety of ages (0–3 years) and lengths (12–430 mm). Our results highlight diversity in the population structure of Redband Trout and provide information valuable to conservation efforts in the Kootenai River system. Interestingly, several tributaries identified as natal habitats of Redband Trout contain substantial obstacles originally considered barriers to upstream fish passage. In addition, some streams in the lower portion of the Kootenai River basin with degraded habitat that were not expected to contribute Redband Trout had natal assignments. These observations indicate previously unknown sources of Redband Trout and support the notion that improved fish passage in disconnected and degraded streams could increase production of Redband Trout throughout the Kootenai River basin. Restoration efforts in the mainstem that include considerations for spawning and natal habitat may also aid in conservation efforts for Redband Trout. Our research demonstrates the value of microchemistry in planning and evaluating habitat restoration to recover fishes in large rivers.

British Columbia, Idaho, Montana↗
USGS2022

Projecting flood frequency curves under near-term climate change

Flood-frequency curves, critical for water infrastructure design, are typically developed based on a stationary climate assumption. However, climate changes are expected to violate this assumption. Here, we propose a new, climate-informed methodology for estimating flood-frequency curves under non-stationary future climate conditions. The methodology develops an asynchronous, semiparametric local-likelihood regression (ASLLR) model that relates moments of annual maximum flood to climate variables using the generalized linear model. We estimate the first two marginal moments (MM) – the mean and variance – of the underlying log-Pearson Type-3 distribution from the ASLLR with the monthly rainfall and temperature as predictors. The proposed methodology, ASLLR-MM, is applied to 40 U.S. Geological Survey streamgages covering 18 water resources regions across the conterminous United States. A correction based on the aridity index was applied on the estimated variance, after which the ASLLR-MM approach was evaluated with both historical (1951–2005) and projected (2006–2035, under RCP4.5 and RCP8.5) monthly precipitation and temperature from eight Global Circulation Models (GCMs) consisting of 39 ensemble members. The estimated flood-frequency quantiles resulting from the ASLLR-MM and GCM members compare well with the flood-frequency quantiles estimated using the historical period of observed climate and flood information for humid basins, whereas the uncertainty in model estimates is higher in arid basins. Considering additional atmospheric and land-surface conditions and a multi-level model structure that includes other basins in a region could further improve the model performance in arid basins.

Water Resources Research↗
USGS2009

Review of the geologic history of the Pontchartrain Basin, northern Gulf of Mexico

The Pontchartrain Basin extends over 44,000 km² from northern Mississippi to the Gulf of Mexico and includes one of the largest and most important estuarine systems in the United States. The basin supports a variety of environments, from woodlands in the north to wetlands in the south, and a growing socioeconomic infrastructure that has led to rapid development of the southern half of the basin over the past two centuries. To properly administer this infrastructure, managers need to understand the complex geologic framework of the basin and how it will respond to continued sea-level rise, variable rates and magnitudes of land subsidence, and human alteration of the landscape. This article summarizes the body of work that describes the regional evolution and stratigraphic architecture of the Pontchartrain Basin. The northern two-thirds of the basin is underlain by a stratigraphy of undifferentiated sands and clays deposited throughout the Plio-Pleistocene by glacially influenced rivers. These deposits were weathered and incised by rivers during sea-level low stands, forming a series of terraces that increase with age from south to north. The southern third of the basin is composed of estuaries formed during the Holocene, while shoreline processes created a series of sandy barriers that restricted communication to the Gulf of Mexico. The Mississippi River completed the geologic development of the basin by building a sequence of subdelta lobes along this southern margin over the past 5000 years, further sealing it from the open Gulf of Mexico. Presently, the modern Mississippi River bypasses the estuarine environment and only contributes sediments during flood events when the river overtops the levee system. Sea-level rise, subsidence within the Holocene delta-plain deposits, and movement along numerous fault systems are the active natural processes that continue to affect basin geomorphology.

Louisiana, Mississippi↗
USGS1974

Generalized geology and structure of the Winkelman 15-minute quadrangle and vicinity, Pinal and Gila Counties, Arizona

A. northwest-trending belt of steeply east-dipping Precambrian and Paleozoic sedimentary rocks extends across the Winkelman 15-minute quadrangle and separates areas of contrasting structures. To the southwest for 60 mi is an expanse of Precambrian basement, largely granite; to the northeast the Precambrian and Paleozoic sedimentary rocks are gently tilted and intricately faulted. The structure within the Winkelman 15-minute quadrangle is interpreted as a monocline separated into en echelon segments by strike-slip and normal faults. Other monoclines lie-to the north and northwest. Most of the tilting that formed the monoclines occurred after deposition of the early(?) Miocene San Manuel Formation. Structural features that repeat the Precambrian and Paleozoic sedimentary rocks within the area and that resemble high-angle faults are believed to have formed as low-angle thrusts, tilted to their present nearly vertical position during development of the monoclines. Structural features that are younger than the monoclines include low-angle gravity slide surfaces which are older than the high-angle normal faults that formed the basin-and-range topography on which the Pliocene sediments were deposited.

Arizona↗
USGS2024

Prioritizing water availability study settings to address geogenic contaminants and related societal factors

Water availability for human and ecological uses depends on both water quantity and water quality. The U.S. Geological Survey (USGS) is developing strategies for prioritizing regional-scale and watershed basin-scale studies of water availability across the nation. Previous USGS ranking processes for basin-scale studies incorporated primarily water quantity factors but are now considering additional water quality factors. This study presents a ranking based on the potential impacts of geogenic constituents on water quality and consideration of societal factors related to water quality. High-concentration geogenic constituents, including trace elements and radionuclides, are among the most prevalent contaminants limiting water availability in the USA and globally. Geogenic constituents commonly occur in groundwater because of subsurface water–rock interactions, and their distributions are controlled by complex geochemical processes. Geogenic constituent mobility can also be affected by human activities (e.g., mining, energy production, irrigation, and pumping). Societal factors and relations to drinking water sources and water quality information are often overlooked when evaluating research priorities. Sociodemographic characteristics, data gaps resulting from historical data-collection disparities, and infrastructure condition/age are examples of factors to consider regarding environmental justice. This paper presents approaches for ranking and prioritizing potential basin-scale study areas across the contiguous USA by considering a suite of conventional physical and geochemical variables related to geogenic constituents, with and without considering variables related to societal factors. Simultaneous consideration of societal and conventional factors could provide decision makers with more diverse, interdisciplinary tools to increase equity and reduce bias in prioritizing focused research areas and future water availability studies.

Environmental Monitoring and Assessment↗
USGS2012

Introduction to special section on the California-Arizona crustal transect: Part II

In May 1988, a multidisciplinary conference was held in Flagstaff, Arizona, to synthesize recent research on the crustal structural and evolution of a transect across the southern Cordillera. This transect extends from the southern Colorado Plateau southwestward across the Arizona Transition Zone, through the Basin and Range province in the Mojave and Sonoran deserts, to the San Andreas fault system, and thence westward across the Imperial Valley and Peninsular Ranges and into the southern California Borderland. Most discussion concerned the eastern part of the transect, from the Colorado Plateau to the San Andreas fault [ Sass et al ., 1988; Howard et al ., 1990]. For this meeting, the name CACTIS (California‐Arizona Crustal Transect‐Interim Synthesis) was coined by J. Sass.

Journal of Geophysical Research↗
USGS1992

Lake trout spawning habitat in the Six Fathom Bank-Yankee Reef lake trout sanctuary, Lake Huron

Attempts to reestablish self-sustaining stocks of lake trout ( Salvelinus namaycush ) in the lower four Great Lakes, where the species was extinguished in the 1950s and 1960s, have been largely unsuccessful. To avoid many of the problems believed to be contributing to this failure, the fishery management community recently established several sanctuaries in the offshore waters of the Great Lakes where the development and protection of self-sustaining stocks of lake trout would be a primary management objective. One of these, the Six Fathom Bank-Yankee Reef sanctuary, was created in the south-central portion of Lake Huron. This sanctuary covers 168,000 ha and includes the shallower portions of the Six Fathom and Ipperwash scarps, which are major bathymetric features in the southern half of the lake. Historical accounts describe Six Fathom Bank as the most important lake trout spawning ground in the lake. Here we present the results of lake bed surveys conducted in the sanctuary with side-scan sonar, underwater videocamera systems, and a small research submarine. Our observations of the lake bed are consistent with what is known of the bedrock stratigraphy, glacial history, and karst geomorphology of the Lake Huron basin. Most of the loose rock we found seemed to be derived from local carbonate bedrock formations, although non-carbonate rock probably from Precambrian sources to the north was also present in some areas. Much of the bedrock and loose rock displayed karst solution features described for the Bruce Peninsula on the Ontario shoreline. Our surveys revealed substantial areas of lake bed at water depths of 20–36 m that resembled suitable spawning and fry production habitat for the shallow-water strains of lake trout that are the focus of the rehabilitation effort. Low mid-lake nutrient levels documented recently by others and the extremely high abundance of Mysis relicta (an important item in the diet of young lake trout) that we documented on Yankee Reef also contributed to our evaluation of the sanctuary as a site with high potential to support a self-sustaining population of lake trout.

Journal of Great Lakes Research↗