Geology topics

Research about Colorado

Source-linked reports with geographic coverage including Colorado.

At least 397 records · Page 22Linked to original sources

USGS2018

40Ar/39Ar geochronology and petrogenesis of the Table Mountain Shoshonite, Golden, Colorado, U.S.A.

The Upper Cretaceous and Lower Paleogene Table Mountain Shoshonite lava flows and their proposed source, the Ralston Buttes intrusions, provide insight into the volcanic history of the Colorado Front Range. This study affirms the long-held hypothesis linking the extrusive Table Mountain lava flows and their intrusive equivalents at Ralston Buttes through major- and trace- element geochemistry. Systematic 40 Ar/ 39 Ar geochronology from all flows and intrusive units refines the eruptive history, improves precision on previously reported ages, and provides tighter constraints on the position of the K-Pg boundary in this location. Four flows are recognized on North and South Table mountains outside of Golden, Colorado. Flow 1 (66.5 ± 0.3 Ma, all ages reported with 2σ uncertainty) is the oldest, most compositionally distinct flow and is separated from younger flows by approximately 35 m of sedimentary deposits of the Denver Formation. Stratigraphically adjacent flows 2 (65.8 ± 0.2 Ma), 3 (65.5 ± 0.3 Ma), and 4 (65.9 ± 0.3 Ma) are compositionally indistinguishable. Lavas (referred to here as unit 5) that form three cone-shaped structures (shown by this study to be volcanic vents of a new unit 5) on top of North Table Mountain are compositionally similar to other units, but yield an age almost 20 m.y. younger (46.94 ± 0.15 Ma). Geochemistry and geochronology suggest that the rim phase of the Ralston plug (65.4 ± 0.2 Ma) is a reasonable source for flows 2, 3, and 4. All units are shoshonites—potassic basalts containing plagioclase, augite, olivine, and magnetite phenocrysts—and plot in the continental-arc field in tectonic discrimination diagrams. A continental-arc setting coupled with Late Cretaceous to early Paleogene ages suggest the high-K magmatism is associated with Laramide tectonism.

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USGS2018

Airborne geophysical characterizationof geologic structure in a mountain headwater system, upper East River, Colorado

Geologic controls on groundwater flow, particularly in tectonically and topographically complex mountainous terrain, can be difficult to quantify without a detailed understanding of the regional subsurface geologic structure. This structure can influence the magnitude of groundwater flow through the mountain block, which in turn impacts groundwater composition and the flux of metals and nutrients to the near - surface ecosystem. In support of several ongoing studies in the upper East River and surrounding w atersheds in central Colorado, regional - scale airborne electromagnetic, magnetic, and radiometric survey s were conducted in late 2017 . These data give a view of the geologic structure underlying the region that is unprecedented in both resolution and spa tial coverage.

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USGS2017

Peak discharge, flood frequency, and peak stage of floods on Big Cottonwood Creek at U.S. Highway 50 near Coaldale, Colorado, and Fountain Creek below U.S. Highway 24 in Colorado Springs, Colorado, 2016

The U.S. Geological Survey (USGS), in cooperation with the Colorado Department of Transportation, determined the peak discharge, annual exceedance probability (flood frequency), and peak stage of two floods that took place on Big Cottonwood Creek at U.S. Highway 50 near Coaldale, Colorado (hereafter referred to as “Big Cottonwood Creek site”), on August 23, 2016, and on Fountain Creek below U.S. Highway 24 in Colorado Springs, Colorado (hereafter referred to as “Fountain Creek site”), on August 29, 2016. A one-dimensional hydraulic model was used to estimate the peak discharge. To define the flood frequency of each flood, peak-streamflow regional-regression equations or statistical analyses of USGS streamgage records were used to estimate annual exceedance probability of the peak discharge. A survey of the high-water mark profile was used to determine the peak stage, and the limitations and accuracy of each component also are presented in this report. Collection and computation of flood data, such as peak discharge, annual exceedance probability, and peak stage at structures critical to Colorado’s infrastructure are an important addition to the flood data collected annually by the USGS. The peak discharge of the August 23, 2016, flood at the Big Cottonwood Creek site was 917 cubic feet per second (ft 3/s ) with a measurement quality of poor (uncertainty plus or minus 25 percent or greater). The peak discharge of the August 29, 2016, flood at the Fountain Creek site was 5,970 ft 3/s with a measurement quality of poor (uncertainty plus or minus 25 percent or greater). The August 23, 2016, flood at the Big Cottonwood Creek site had an annual exceedance probability of less than 0.01 (return period greater than the 100-year flood) and had an annual exceedance probability of greater than 0.005 (return period less than the 200-year flood). The August 23, 2016, flood event was caused by a precipitation event having an annual exceedance probability of 1.0 (return period of 1 year, or the 1-year storm), which is a statistically common (high probability) storm. The Big Cottonwood Creek site is downstream from the Hayden Pass Fire burn area, which dramatically altered the hydrology of the watershed and caused this statistically rare (low probability) flood from a statistically common (high probability) storm. The peak flood stage at the cross section closest to the U.S. Highway 50 culvert was 6,438.32 feet (ft) above the North American Datum of 1988 (NAVD 88). The August 29, 2016, flood at the Fountain Creek site had an estimated annual exceedance probability of 0.5505 (return period equal to the 1.8-year flood). The August 29, 2016, flood event was caused by a precipitation event having an annual exceedance probability of 1.0 (return period of 1 year, or the 1-year storm). The peak stage during this flood at the cross section closest to the U.S. Highway 24 bridge was 5,832.89 ft (NAVD 88). Slope-area indirect discharge measurements were carried out at the Big Cottonwood Creek and Fountain Creek sites to estimate peak discharge of the August 23, 2016, flood and August 29, 2016, flood, respectively. The USGS computer program Slope-Area Computation Graphical User Interface was used to compute the peak discharge by adding the surveyed cross sections with Manning roughness coefficient assignments to the high-water marks. The Manning roughness coefficients for each cross section were estimated in the field using the Cowan method.

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USGS2017

Magnetotelluric imaging of lower crustal melt and lithospheric hydration in the Rocky Mountain Front transition zone, Colorado, USA

We present an electrical resistivity model of the crust and upper mantle from two‐dimensional (2‐D) anisotropic inversion of magnetotelluric data collected along a 450 km transect of the Rio Grande rift, southern Rocky Mountains, and High Plains in Colorado, USA. Our model provides a window into the modern‐day lithosphere beneath the Rocky Mountain Front to depths in excess of 150 km. Two key features of the 2‐D resistivity model are (1) a broad zone (~200 km wide) of enhanced electrical conductivity (<20 Ωm) in the midcrust to lower crust that is centered beneath the highest elevations of the southern Rocky Mountains and (2) hydrated lithospheric mantle beneath the Great Plains with water content in excess of 100 ppm. We interpret the high conductivity region of the lower crust as a zone of partially molten basalt and associated deep‐crustal fluids that is the result of recent (less than 10 Ma) tectonic activity in the region. The recent supply of volatiles and/or heat to the base of the crust in the late Cenozoic implies that modern‐day tectonic activity in the western United States extends to at least the western margin of the Great Plains. The transition from conductive to resistive upper mantle is caused by a gradient in lithospheric modification, likely including hydration of nominally anhydrous minerals, with maximum hydration occurring beneath the Rocky Mountain Front. This lithospheric “hydration front” has implications for the tectonic evolution of the continental interior and the mechanisms by which water infiltrates the lithosphere.

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USGS2017

Geologic map of the upper Arkansas River valley region, north-central Colorado

This 1:50,000-scale U.S. Geological Survey geologic map represents a compilation of the most recent geologic studies of the upper Arkansas River valley between Leadville and Salida, Colorado. The valley is structurally controlled by an extensional fault system that forms part of the prominent northern Rio Grande rift, an intra-continental region of crustal extension. This report also incorporates new detailed geologic mapping of previously poorly understood areas within the map area and reinterprets previously studied areas. The mapped region extends into the Proterozoic metamorphic and intrusive rocks in the Sawatch Range west of the valley and the Mosquito Range to the east. Paleozoic rocks are preserved along the crest of the Mosquito Range, but most of them have been eroded from the Sawatch Range. Numerous new isotopic ages better constrain the timing of both Proterozoic intrusive events, Late Cretaceous to early Tertiary intrusive events, and Eocene and Miocene volcanic episodes, including widespread ignimbrite eruptions. The uranium-lead ages document extensive about 1,440-million years (Ma) granitic plutonism mostly north of Buena Vista that produced batholiths that intruded an older suite of about 1,760-Ma metamorphic rocks and about 1,700-Ma plutonic rocks. As a result of extension during the Neogene and possibly latest Paleogene, the graben underlying the valley is filled with thick basin-fill deposits (Dry Union Formation and older sediments), which occupy two sub-basins separated by a bedrock high near the town of Granite. The Dry Union Formation has undergone deep erosion since the late Miocene or early Pliocene. During the Pleistocene, ongoing stream incision by the Arkansas River and its major tributaries has been interrupted by periodic aggradation. From Leadville south to Salida as many as seven mapped alluvial depositional units, that range in age from early to late Pleistocene, record periodic aggradational events along these streams that are commonly associated with deposition of glacial outwash or bouldery glacial-flood deposits. Many previously unrecognized Neogene and Quaternary faults, some of the latter with possible Holocene displacement, have been identified on lidar (light detection and ranging) imagery which covers 59 percent of the map area. This imagery has also permitted more accurate remapping of glacial, fluvial, and mass-movement deposits and aided in the determination of their relative ages. Recently published 10 beryllium cosmogenic surface-exposure ages, coupled with our new geologic mapping, have revealed the timing and rates of late Pleistocene deglaciation. Glacial dams that impounded the Arkansas River at Clear Creek and possibly at Pine Creek failed at least three times during the middle and late Pleistocene, resulting in catastrophic floods and deposition of enormous boulders and bouldery alluvium downstream; at least two failures occurred during the late Pleistocene during the Pinedale glaciation.

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USGS2017

Riparian bird density decline in response to biocontrol of Tamarix from riparian ecosystems along the Dolores River in SW Colorado, USA

Biocontrol of invasive tamarisk ( Tamarix spp.) in the arid Southwest using the introduced tamarisk beetle ( Diorhabda elongata ) has been hypothesized to negatively affect some breeding bird species, but no studies to date have documented the effects of beetle-induced defoliation on riparian bird abundance. We assessed the effects of tamarisk defoliation by monitoring defoliation rates, changes in vegetation composition, and changes in density of six obligate riparian breeding bird species at two sites along the Dolores River in Colorado following the arrival of tamarisk beetles. We conducted bird point counts from 2010 to 2014 and modeled bird density as a function of native vegetation density and extent of defoliation using hierarchical distance sampling. Maximum annual defoliation decreased throughout the study period, peaking at 32–37% in 2009–2010 and dropping to 0.5–15% from 2011–2014. Stem density of both tamarisk and native plants declined throughout the study period until 2014. Density of all bird species declined throughout most of the study, with Song Sparrow disappearing from the study sites after 2011. Blue Grosbeak, Yellow-breasted Chat, and Yellow Warbler densities were negatively related to defoliation in the previous year, while Lazuli Bunting exhibited a positive relationship with defoliation. These findings corroborate earlier predictions of species expected to be sensitive to defoliation as a result of nest site selection. Tamarisk defoliation thus had short-term negative impacts on riparian bird species; active restoration may be needed to encourage the regrowth of native riparian vegetation, which in the longer-term may result in increased riparian bird density.

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USGS2017

Description of chronostratigraphic units preserved as channel deposits and geomorphic processes following a basin-scale disturbance by a wildfire in Colorado

The consequence of a 1996 wildfire disturbance and a subsequent high-intensity summer convective rain storm (about 110 millimeters per hour) was the deposition of a sediment superslug in the Spring Creek basin (26.8 square kilometers) of the Front Range Mountains in Colorado. Spring Creek is a tributary to the South Platte River upstream from Strontia Springs Reservoir, which supplies domestic water for the cities of Denver and Aurora. Changes in a superslug were monitored over the course of 18 years (1996–2014) by repeat surveys at 18 channel cross sections spaced at nearly equal intervals along a 1,500-meter study reach and by a time series of photographs of each cross section. Surveys were not repeated at regular time intervals but after major changes caused by different geomorphic processes. The focus of this long-term study was to understand the evolution and internal alluvial architecture of chronostratigraphic units (defined as the volume of sediment deposited between two successive surveys), and the preservation or storage of these units in the superslug. The data are presented as a series of 18 narratives (one for each cross section) that summarize the changes, illustrate these changes with photographs, and provide a preservation plot showing the amount of each chronostratigraphic unit still remaining in June 2014. The most significant hydrologic change after the wildfire was an exponential decrease in peak discharge of flash floods caused by summer convective rain storms. In response to these hydrologic changes, all 18 locations went through an aggradation phase, an incision phase, and finally a stabilization phase. However, the architecture of the chronostratigraphic units differs from cross section to cross section, and units are characterized by either a laminar, fragmented, or hybrid alluvial architecture. In response to the decrease in peak-flood discharge and the increase in hillslope and riparian vegetation, Spring Creek abandoned many of the nearly horizontal erosional and depositional surfaces and left a landscape consisting of a series of cut-and-fill terraces as a legacy of this wildfire disturbance.

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USGS2017

Effects of an extreme flood on trace elements in river water—From urban stream to major river basin

Major floods adversely affect water quality through surface runoff, groundwater discharge, and damage to municipal water infrastructure. Despite their importance, it can be difficult to assess the effects of floods on streamwater chemistry because of challenges collecting samples and the absence of baseline data. This study documents water quality during the September 2013 extreme flood in the South Platte River, Colorado, USA. Weekly time-series water samples were collected from 3 urban source waters (municipal tap water, streamwater, and wastewater treatment facility effluent) under normal-flow and flood conditions. In addition, water samples were collected during the flood at 5 locations along the South Platte River and from 7 tributaries along the Colorado Front Range. Samples were analyzed for 54 major and trace elements. Specific chemical tracers, representing different natural and anthropogenic sources and geochemical behaviors, were used to compare streamwater composition before and during the flood. The results differentiate hydrological processes that affected water quality: (1) in the upper watershed, runoff diluted most dissolved constituents, (2) in the urban corridor and lower watershed, runoff mobilized soluble constituents accumulated on the landscape and contributed to stream loading, and (3) flood-induced groundwater discharge mobilized soluble constituents stored in the vadose zone.

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USGS2017

Characterization of water quality and suspended sediment during cold-season flows, warm-season flows, and stormflows in the Fountain and Monument Creek watersheds, Colorado, 2007–2015

From 2007 through 2015, the U.S. Geological Survey, in cooperation with Colorado Springs City Engineering, conducted a study in the Fountain and Monument Creek watersheds, Colorado, to characterize surface-water quality and suspended-sediment conditions for three different streamflow regimes with an emphasis on characterizing water quality during storm runoff. Data collected during this study were used to evaluate the effects of stormflows and wastewater-treatment effluent discharge on Fountain and Monument Creeks in the Colorado Springs, Colorado, area. Water-quality samples were collected at 2 sites on Upper Fountain Creek, 2 sites on Monument Creek, 3 sites on Lower Fountain Creek, and 13 tributary sites during 3 flow regimes: cold-season flow (November–April), warm-season flow (May–October), and stormflow from 2007 through 2015. During 2015, additional samples were collected and analyzed for Escherichia coli ( E. coli ) during dry weather conditions at 41 sites, located in E. coli impaired stream reaches, to help identify source areas and scope of the impairment. Concentrations of E. coli , total arsenic, and dissolved copper, selenium, and zinc in surface-water samples were compared to Colorado in-stream standards. Stormflow concentrations of E. coli frequently exceeded the recreational use standard of 126 colonies per 100 milliliters at main-stem and tributary sites by more than an order of magnitude. Even though median E. coli concentrations in warm-season flow samples were lower than median concentrations in storm-flow samples, the water quality standard for E. coli was still exceeded at most main-stem sites and many tributary sites during warm-season flows. Six samples (three warm-season flow and three stormflow samples) collected from Upper Fountain Creek, upstream from the confluence of Monument Creek, and two stormflow samples collected from Lower Fountain Creek, downstream from the confluence with Monument Creek, exceeded the acute water-quality standard for total arsenic of 50 micrograms per liter. All concentrations of dissolved copper, selenium, and zinc measured in samples were below the water-quality standard. Concentrations of dissolved nitrate plus nitrite generally increased from upstream to downstream during all flow periods. The largest downstream increase in dissolved nitrate plus nitrite concentration was measured between sites 07103970 and 07104905 on Monument Creek. All but one tributary that drain into Monument Creek between the two sites had higher median nitrate plus nitrite concentrations than the nearest upstream site on Monument Creek, site 07103970 (MoCr_Woodmen). Increases in the concentration of dissolved nitrate plus nitrite were also evident below wastewater treatment plants located on Fountain Creek. Most stormflow concentrations of dissolved trace elements were smaller than concentrations from cold-season flow or warm-season samples. However, median concentrations of total arsenic, lead, manganese, nickel, and zinc generally were much larger during periods of stormflow than during cold-season flow or warm-season fl. Median concentrations of total arsenic, total copper, total lead, dissolved and total manganese, total nickel, dissolved and total selenium, and dissolved and total zinc concentrations increased from 1.5 to 28.5 times from site 07103700 (FoCr_Manitou) to 07103707 (FoCr_8th) during cold-season and warm-season flows, indicating a large source of trace elements between these two sites. Both of these sites are located on Fountain Creek, upstream from the confluence with Monument Creek. Median suspended-sediment concentrations and median suspended-sediment loads increased in the downstream direction during all streamflow regimes between Monument Creek sites 07103970 (MoCr_Woodmen) and 07104905 (MoCr_Bijou); however, statistically significant increase (p-value less than 0.05) were only present during warm-season flow and stormflow. Significant increases in median suspended sediment concentrations were measured during cold-season flow and warm-season flow between Upper Fountain Creek site 07103707 (FoCr_8th) and Lower Fountain Creek site 07105500 (FoCr_Nevada) because of inflows from Monument Creek with higher suspended-sediment concentrations. Median suspended-sediment concentrations between sites 07104905 (MoCr_Bijou) and 07105500 (FoCr_Nevada) increased significantly during warm-season flow but showed no significant differences during cold-season flow and stormflow. Significant decreases in median suspended-sediment concentrations were measured between sites 07105500 (FoCr_Nevada) and 07105530 (FoCr_Janitell) during all flow regimes. Suspended-sediment concentrations, discharges, and yields associated with stormflow were significantly larger than those associated with warm-season flow. Although large spatial variations in suspended-sediment yields occurred during warm-season flows, the suspended-sediment yield associated with stormflow were as much as 1,000 times larger than the suspended-sediment yields that occurred during warm-season flow.

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USGS2017

Prediction of forest canopy and surface fuels from Lidar and satellite time series data in a bark beetle-affected forest

Wildfire behavior depends on the type, quantity, and condition of fuels, and the effect that bark beetle outbreaks have on fuels is a topic of current research and debate. Remote sensing can provide estimates of fuels across landscapes, although few studies have estimated surface fuels from remote sensing data. Here we predicted and mapped field-measured canopy and surface fuels from light detection and ranging (lidar) and Landsat time series explanatory variables via random forest (RF) modeling across a coniferous montane forest in Colorado, USA, which was affected by mountain pine beetles ( Dendroctonus ponderosae Hopkins) approximately six years prior. We examined relationships between mapped fuels and the severity of tree mortality with correlation tests. RF models explained 59%, 48%, 35%, and 70% of the variation in available canopy fuel, canopy bulk density, canopy base height, and canopy height, respectively (percent root-mean-square error (%RMSE) = 12–54%). Surface fuels were predicted less accurately, with models explaining 24%, 28%, 32%, and 30% of the variation in litter and duff, 1 to 100-h, 1000-h, and total surface fuels, respectively (%RMSE = 37–98%). Fuel metrics were negatively correlated with the severity of tree mortality, except canopy base height, which increased with greater tree mortality. Our results showed how bark beetle-caused tree mortality significantly reduced canopy fuels in our study area. We demonstrated that lidar and Landsat time series data contain substantial information about canopy and surface fuels and can be used for large-scale efforts to monitor and map fuel loads for fire behavior modeling at a landscape scale.

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USGS2017

Evidence of compounded disturbance effects on vegetation recovery following high-severity wildfire and spruce beetle outbreak

Spruce beetle ( Dendroctonus rufipennis ) outbreaks are rapidly spreading throughout subalpine forests of the Rocky Mountains, raising concerns that altered fuel structures may increase the ecological severity of wildfires. Although many recent studies have found no conclusive link between beetle outbreaks and increased fire size or canopy mortality, few studies have addressed whether these combined disturbances produce compounded effects on short-term vegetation recovery. We tested for an effect of spruce beetle outbreak severity on vegetation recovery in the West Fork Complex fire in southwestern Colorado, USA, where much of the burn area had been affected by severe spruce beetle outbreaks in the decade prior to the fire. Vegetation recovery was assessed using the Landsat-derived Normalized Difference Vegetation Index (NDVI) two years after the fire, which occurred in 2013. Beetle outbreak severity, defined as the basal area of beetle-killed trees within Landsat pixels, was estimated using vegetation index differences (dVIs) derived from pre-outbreak and post-outbreak Landsat images. Of the seven dVIs tested, the change in Normalized Difference Moisture Index (dNDMI) was most strongly correlated with field measurements of beetle-killed basal area (R 2 = 0.66). dNDMI was included as an explanatory variable in sequential autoregressive (SAR) models of NDVI 2015 . Models also included pre-disturbance NDVI, topography, and weather conditions at the time of burning as covariates. SAR results showed a significant correlation between NDVI 2015 and dNDMI, with more severe spruce beetle outbreaks corresponding to reduced post-fire vegetation cover. The correlation was stronger for models which were limited to locations in the red stage of outbreak (outbreak ≤ 5 years old at the time of fire) than for models of gray-stage locations (outbreak > 5 years old at the time of fire). These results indicate that vegetation recovery processes may be negatively impacted by severe spruce beetle outbreaks occurring within a decade of stand-replacing wildfire.

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USGS2017

Precipitation, streamflow, suspended-sediment, and water-quality data for the U.S. Army Garrison Fort Carson and Piñon Canyon Maneuver Site, Colorado, 1966–2015

The U.S. Army Garrison Fort Carson (AGFC) and the Piñon Canyon Maneuver Site (PCMS) are facilities operated by the U.S. Department of the Army in southern Colorado. The U.S. Geological Survey, in cooperation with the U.S. Department of the Army, established a hydrologic and water-quality data-collection network at the AGFC in June 1978 and at the PCMS in October 1982. The data-collection networks are designed to assess the quantity and quality of water resources and monitor the effects of military training activities on streamflow and water quality. Two preexisting U.S. Geological Survey streamgages at the PCMS were incorporated into the data-collection network at the time it was established, providing periods of record that begin as early as 1966. This report presents and summarizes precipitation, streamflow, suspended-sediment, and water-quality data from 34 U.S. Geological Survey sites on or near the AGFC and the PCMS for the period of record at each site. (Streamflow data are presented as discharge in cubic feet per second.) At AGFC, daily sum precipitation ranged from 0 to 11.85 inches, daily mean discharge ranged from 0 to 836 cubic feet per second, and daily mean suspended-sediment discharge ranged from 0 to 39,900 tons per day. With the exception of total (unfiltered) mercury and filtered sulfate at two sites and filtered manganese at three sites, 95th percentile trace element concentrations and median total (unfiltered) metal concentrations were less than regulatory numeric standards for all samples. However, individual water-quality results occasionally exceeded respective regulatory numeric standards. At the PCMS, daily sum precipitation ranged from 0 to 4.59 inches, daily mean discharge ranged from 0 to 4,190 cubic feet per second, and daily mean suspended-sediment discharge ranged from 0 to 21,100 tons per day. Water-quality results, 95th percentile trace element concentrations, and median total (unfiltered) metal concentrations were less than regulatory numeric standards for most properties and constituents except for filtered chloride at one site, filtered sulfate at six sites, filtered phosphorus at one site, filtered manganese at three sites, and total (unfiltered) iron at three sites. Individual water-quality values also occasionally exceeded respective regulatory numeric standards.

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USGS2017

Estimation of salt loads for the Dolores River in the Paradox Valley, Colorado, 1980–2015

Regression models that relate total dissolved solids (TDS) concentrations to specific conductance were used to estimate salt loads for two sites on the Dolores River in the Paradox Valley in western Colorado. The salt-load estimates will be used by the Bureau of Reclamation to evaluate salt loading to the river coming from the Paradox Valley and the effect of the Paradox Valley Unit (PVU), a project designed to reduce the salinity of the Colorado River. A second-order polynomial provided the best fit of the discrete data for both sites on the river. The largest bias occurred in samples with elevated sulfate concentrations (greater than 500 milligrams per liter), which were associated with short-duration runoff events in late summer and fall. Comparison of regression models from a period of time before operation began at the PVU and three periods after operation began suggests the relation between TDS and specific conductance has not changed over time. Net salt gain through the Paradox Valley was estimated as the TDS load at the downstream site minus the load at the upstream site. The mean annual salt gain was 137,900 tons per year prior to operation of the PVU (1980–1993) and 43,300 tons per year after the PVU began operation (1997–2015). The difference in annual salt gain in the river between the pre-PVU and post-PVU periods was 94,600 tons per year, which represents a nearly 70 percent reduction in salt loading to the river.

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USGS2017

Tributary use by imperiled Flannelmouth and Bluehead Suckers in the upper Colorado River Basin

Habitat alterations and establishment of nonnative fishes have reduced the distributions of Flannelmouth Sucker Catostomus latipinnis and Bluehead Sucker C. discobolus to less than 50% of their historical ranges in the Colorado River basin. Tributaries are sometimes less altered than main-stem habitat in the basin and may be important to support various life history processes, but their role in the maintenance of Flannelmouth Sucker and Bluehead Sucker populations is poorly understood. Using mark–recapture techniques, we show tributaries are important habitat for native suckers in the upper Colorado River basin and report three main findings. First, both Flannelmouth and Bluehead suckers likely respond to a thermal cue that initiates spawning movement patterns. Suckers moved into Coal Creek from the White River beginning in mid-May of 2012 and 2013 to spawn. The majority of sucker spawning movements occurred when water temperatures in White River exceeded 11–14°C and those in Coal Creek were 2.5–4°C warmer, while flows varied between years. Second, based on PIT tag detection arrays, 13–45% of suckers showed spawning site fidelity. Sampling only with fyke nets would have resulted in the conclusion that site fidelity by native suckers was only 1–17%, because nets were less efficient at detecting marked fish. Third, most suckers of both species emigrated from Coal Creek within 48 h after being captured while suckers that were detected only via arrays remained resident for 10–12 d. The posthandling flight response we observed was not anticipated and to our knowledge has not been previously reported for these species. Remote PIT tag antenna arrays allowed for a stronger inference regarding movement and tributary use by these species than what could be achieved using just fyke nets. Tributaries are an important part of Flannelmouth Sucker and Bluehead Sucker life history and thus important to conservation strategies for these species.

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USGS2017

U-Pb ages and geochemistry of zircon from Proterozoic plutons of the Sawatch and Mosquito ranges, Colorado, U.S.A.: Implications for crustal growth of the central Colorado province

A broad study of zircons from plutonic rocks of the Sawatch and Mosquito ranges of west-central Colorado (U.S.A.) was undertaken to significantly refine the magmatic chronology and chemistry of this under-studied region of the Colorado province. This region was chosen because it lies just to the north of the suspected arc-related Gunnison-Salida volcano-plutonic terrane, which has been the subject of many recent investigations—and whose origin is still debated. Our new results provide important insights into the processes active during Proterozoic crustal evolution in this region, and they have important ramifications for broader-scope crustal evolution models for southwestern North America. Twenty-four new U-Pb ages and sequentially acquired rare-earth element (REE), U, Th, and Hf contents of zircon have been determined using the sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG). These zircon geochemistry data, in conjunction with whole-rock major- and trace-element data, provide important insights into zircon crystallization and melt fractionation, and they help to further constrain the tectonic environment of magma generation. Our detailed zircon and whole-rock data support the following three interpretations: (1) The Roosevelt Granite in the southern Sawatch Range was the oldest rock dated at 1,766 ± 7 Ma, and it intruded various metavolcanic and metasedimentary rocks. Geochemistry of both whole-rock and zircon supports the contention that this granite was produced in a magmatic arc environment and, therefore, is likely an extension of the older Dubois Greenstone Belt of the Gunnison Igneous Complex (GIC) and the Needle Mountains (1,770–1,755 Ma). Rocks of the younger Cochetopa succession of the GIC, the Salida Greenstone Belt, and the Sangre de Cristo Mountains (1,740–1,725 Ma) were not found in the Sawatch and Mosquito ranges. This observation strongly suggests that the northern edge of the Gunnison-Salida arc terrane underlies the southern portion of the Sawatch and Mosquito ranges. (2) Calc-alkalic to alkali-calcic magmas intruded this region approximately 55 m.y. after the Roosevelt Granite with emplacement of pre-deformational plutons at ca. 1,710 Ma (e.g., Henry Mountain Granite and diorite of Denny Creek), and this continued for at least 30 m.y., ending with emplacement of post-deformational plutons at ca. 1,680 Ma (e.g., Kroenke Granodiorite, granite of Fairview Peak, and syenite of Mount Yale). The timing of deformation can be constrained to sometime after intrusion of the diorite of Denny Creek and likely before the emplacement of the undeformed granite of Fairview Peak. Geochemistry of both whole-rock and zircon indicates that the older group of ca. 1,710-Ma plutons formed at shallower depths, and then they intruded the younger group of more deeply generated, commonly peraluminous and sodic plutons. Although absent in the Sawatch and Mosquito ranges, Mazatzal-age (ca. 1,680–1,620 Ma) plutonic rocks are present regionally. Inherited zircon components of Mazatzal-age were found as cores in some 1.4-Ga Sawatch and Mosquito Range zircons, indicating the likelihood of a relatively local source. These combined data suggest the possibility that all were produced within a continental-margin magmatic arc created as a result of southward-migrating (slab rollback?), north-dipping subduction to the south of the region. (3) Widespread Mesoproterozoic plutonism—with emplacement at various depths and exhibiting bimodal geochemistry—is recognized in 16 different samples. An older group of predominantly peraluminous, yet magnesian granitoids (e.g., granodiorite of Sayers, granite of Taylor River, and the St. Kevin Granite) were emplaced between ca. 1,450 and 1,425 Ma. These geochemical parameters suggest moderate degrees of partial melting in a low-pressure environment. Three younger metaluminous, but ferroan plutons (diorite of Grottos, diorite of Mount Elbert, and granodiorite of Mount Harvard), probably represent a final magmatic pulse at ca. 1,416 Ma. A comprehensive treatment of zircon REE and whole-rock trace-element behavior from Proterozoic rocks is scarce. Discriminant U/Yb versus Y diagrams using zircon data show that the Sawatch and Mosquito plutons are of continental origin, not oceanic. Additional bivariate diagrams incorporating cation ratio combinations of Gd, Ce, Yb, U, Th, Hf, and Eu offer refined insight into differences in fractionation trends and depth of magma generation for the various plutons. These interpretations, on the basis of zircon trace-element data, are mirrored in the whole-rock geochemistry data.

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USGS2017

Burrow dusting or oral vaccination prevents plague-associated prairie dog colony collapse

Plague impacts prairie dogs ( Cynomys spp.), the endangered black-footed ferret ( Mustela nigripes ) and other sensitive wildlife species. We compared efficacy of prophylactic treatments (burrow dusting with deltamethrin or oral vaccination with recombinant “sylvatic plague vaccine” [RCN-F1/V307]) to placebo treatment in black-tailed prairie dog ( C. ludovicianus ) colonies. Between 2013 and 2015, we measured prairie dog apparent survival, burrow activity and flea abundance on triplicate plots (“blocks”) receiving dust, vaccine or placebo treatment. Epizootic plague affected all three blocks but emerged asynchronously. Dust plots had fewer fleas per burrow ( P < 0.0001), and prairie dogs captured on dust plots had fewer fleas ( P < 0.0001) than those on vaccine or placebo plots. Burrow activity and prairie dog density declined sharply in placebo plots when epizootic plague emerged. Patterns in corresponding dust and vaccine plots were less consistent and appeared strongly influenced by timing of treatment applications relative to plague emergence. Deltamethrin or oral vaccination enhanced apparent survival within two blocks. Applying insecticide or vaccine prior to epizootic emergence blunted effects of plague on prairie dog survival and abundance, thereby preventing colony collapse. Successful plague mitigation will likely entail strategic combined uses of burrow dusting and oral vaccination within large colonies or colony complexes.

Colorado↗

USGS2017

Transient electromagnetic soundings in the San Luis Valley, Colorado, near the Great Sand Dunes National Park and Preserve and the Alamosa National Wildlife Refuge (field seasons 2007, 2009, and 2011)

Transient electromagnetic (TEM) soundings were made in the San Luis Valley, Colorado, to map the location of a blue clay unit as well as to investigate the presence of suspected faults. A total of 147 soundings were made near and in Great Sand Dunes National Park and Preserve, and an additional 6 soundings were made near Hansen Bluff on the eastern edge of the Alamosa National Wildlife Refuge. The blue clay is a significant hydrologic feature in the area that separates an unconfined surface aquifer from a deeper confined aquifer. Knowledge of its location is important to regional hydrological models. Previous analysis of well logs has shown that the blue clay has a resistivity of 10 ohm-meters or less, which is in contrast to the higher resistivity of sand, gravel, and other clay units found in the area, making it a very good target for TEM soundings. The top of the blue clay was found to have considerable relief, suggesting the possibility of deformation of the clay during or after deposition. Because of rift activity, deformation is to be expected. Of the TEM profiles made across faults identified by aeromagnetic data, some showed resistivity variations and (or) subsurface elevation relief of resistivity units, suggestive of faulting. Such patterns were not associated with all suspected faults. The Hansen Bluff profile showed variations in resistivity and depth to conductor that coincide with a scarp between the highlands to the east and the floodplain of the Rio Grande to the west.

Colorado↗

USGS2017

Potential human impacts of overlapping land-use and climate in a sensitive dryland: a case study of the Colorado Plateau, USA

Climate and land-use interactions are likely to affect future environmental and socioeconomic conditions in drylands, which tend to be limited by water resources and prone to land degradation. We characterized the potential for interactions between land-use types and land-use and climate change in a model dryland system, the Colorado Plateau, a region with a history of climatic variability and land-use change. We analyzed the spatial and temporal trends in aridification, land-use, and recreation at the county and 10 km2 grid scales. Our results show that oil and gas development and recreation may interact due to increasing trends and overlapping areas of high intensity. Projections suggest that aridification will impact all vegetation classes, with some of the highest proportional change in the south-east. The results suggest that the rate of change and spatial pattern of land-use in the future may differ from past patterns in land-use scale and intensity.

Colorado↗