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USGS2015

Desert grassland responses to climate and soil moisture suggest divergent vulnerabilities across the southwestern United States

Climate change predictions include warming and drying trends, which are expected to be particularly pronounced in the southwestern United States. In this region, grassland dynamics are tightly linked to available moisture, yet it has proven difficult to resolve what aspects of climate drive vegetation change. In part, this is because it is unclear how heterogeneity in soils affects plant responses to climate. Here, we combine climate and soil properties with a mechanistic soil water model to explain temporal fluctuations in perennial grass cover, quantify where and the degree to which incorporating soil water dynamics enhances our ability to understand temporal patterns, and explore the potential consequences of climate change by assessing future trajectories of important climate and soil water variables. Our analyses focused on long-term (20–56 years) perennial grass dynamics across the Colorado Plateau, Sonoran, and Chihuahuan Desert regions. Our results suggest that climate variability has negative effects on grass cover, and that precipitation subsidies that extend growing seasons are beneficial. Soil water metrics, including the number of dry days and availability of water from deeper (>30 cm) soil layers, explained additional grass cover variability. While individual climate variables were ranked as more important in explaining grass cover, collectively soil water accounted for 40–60% of the total explained variance. Soil water conditions were more useful for understanding the responses of C 3 than C 4 grass species. Projections of water balance variables under climate change indicate that conditions that currently support perennial grasses will be less common in the future, and these altered conditions will be more pronounced in the Chihuahuan Desert and Colorado Plateau. We conclude that incorporating multiple aspects of climate and accounting for soil variability can improve our ability to understand patterns, identify areas of vulnerability, and predict the future of desert grasslands.

Global Change Biology↗

USGS1979

The petrogenesis and trace-element geochemistry of intermediate lavas from Humphreys Peak, San Francisco volcanic field, Arizona

The San Francisco Mountain lavas exposed in the upper portion of the southeast slope of Humphreys Peak are composed of three petrographically distinct types: (1) a lower series of hornblende pyroxene andesites; (2) a group of hypersthene dacites; and (3) an upper series of olivine andesites. These rocks have been shown to be related by crystallization differentiation through comparison of groundmass and bulk rock-chemical data. The major and trace elements from rocks representative of the volcanic field form continuous trends, an indication of differentiation rather than separate and discrete magma sources. Xenoliths found in the volcanic field are composed of the proper major and trace element contents to allow their extraction from a primary melt to form residual melts of more silicic lavas. The alkali olivine basalts, probably formed by partial melting of the mantle at a depth of 35-60 km, may well have differentiated to the alkali-rich highalumina basalts at depths of 15-35 km (the lower crust of the Colorado Plateau) by removal of olivine and clinopyroxene. Some of both lava types erupted periodically onto the surface while others continued to differentiate. Removal of plagioclase, with more minor amounts of olivine and pyroxenes (of less mafic composition than those above), from the high-alumina basalts eventually may have resulted in magmas similar in composition to the intermediate and more silicic rocks. The intermediate and silicic magmas contain hornblende and probably formed between 5 and 8 km. This places a minimum depth of penetration by the normal faults, such as the Mesa Butte fault, along which many silicic centers are aligned, at a depth of 5-8 km. Because of the continuous linear trends of the chemical data and the repetition over the past 6 m.y. of most of the rock types, the mantle beneath this southwestern margin of the Colorado Plateau apparently has not changed significantly during the past 6 m.y., nor is it likely to be very heterogeneous.

Arizona↗

USGS1967

Preliminary geologic section from Pahute Mesa, Nevada Test Site, to Enterprise, Utah

The 154-mile long geologic cross section trends nearly perpendicular to the structural grain of the Basin-Range province in Nevada, and in Utah extends eastward into the transition zone between the Basin-Range and Colorado Plateau provinces. The structure is characterized by complex thrust: faults, involving uppermost Precambrian to lower Mesozoic sedimentary rocks, and normal faults which cut: the thick sequence of Tertiary volcanic rocks as well as older rocks. Some of the normal faults are the result of caldera collapse. The principal normal faults trend northerly west: of Delamar, Nev., and form north-trending basins and ranges. Farther east the principal faults trend northwesterly, and form a moderately rugged highland rather than distinct basins and ranges. The uppermost Precambrian-Paleozoic strata thin markedly eastward across the region. The pre-Pennsylvanian sedimentary rocks vary from 32,500 feet: in thickness at the Nevada Test: Site (Harley Barnes, E. N. Hinrichs, F. A. McKeown and P. P. Orkild, written commun., 1963) to 4,500 feet: in the Beaver Dam Mountains in western Utah (Cook, 1960). Thick Mesozoic deposits, similar to those of the Colorado Plateau, are present in western Utah, but are represented in eastern Nevada by only thin patches of Triassic rock.

Nevada, Utah↗

USGS2020

The right trait in the right place at the right time: Matching traits to environment improves restoration outcomes

(Munson) The challenges of restoration in dryland ecosystems are growing due to a rise in anthropogenic disturbance and increasing aridity. Plant functional traits are often used to predict plant performance and can offer a window into the potential outcomes of restoration efforts across environmental gradients. We tracked 15 years of seeding outcomes across 150 sites on the Colorado Plateau, a cold desert ecoregion in the western United States, and analyzed the independent and interactive effects of functional traits (seed mass, height, and specific leaf area) and local biologically relevant climate variables on seeding success. We predicted that the best models would include an interaction between plant traits and climate, indicating a need to match the right trait value to the right climate conditions in order to maximize seeding success. Indeed, we found that both plant height and seed size significantly interacted with temperature seasonality, with larger seeds and taller plants performing better in more seasonal environments. We also determined that these trait-environment patterns are not driven by the use of native vs. non-native species. Our results lend insight to using plant traits to inform the selection of seed mixes for restoring areas with specific climatic conditions, while also demonstrating the strong influence of temperature seasonality on seeding success in the Colorado Plateau region.

Colorado Plateau↗

USGS1996

The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation

The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic inversions of the P wave travel time data are given: an Aki-Christofferson-Husebye (ACH) block model inversion and a downward projection inversion. The tomographic inversions reveal a NE-SW to NNE-SSW trending feature at depths of 35 to 145 km with a velocity reduction of 7 to 8% relative to mantle velocities beneath the Great Plains. This region correlates with the transition zone between the Colorado Plateau and the Rio Grande rift and is bounded on the NW by the Jemez lineament, a N52°E trending zone of late Miocene to Holocene volcanism. S wave delays plotted against P wave delays are fit with a straight line giving a slope of 3.0 ± 0.4. This correlation and the absolute velocity reduction imply that temperatures in the lithosphere are close to the solidus, consistent with, but not requiring, the presence of partial melt in the mantle beneath the Rio Grande rift. The attenuation data could imply the presence of partial melt. We compare our results with other geophysical and geologic data. We propose that any north-south trending thermal (velocity) anomaly that may have existed in the upper mantle during earlier (Oligocene to late Miocene) phases of rifting and that may have correlated with the axis of the rift has diminished with time and has been overprinted with more recent structure. The anomalously low-velocity body presently underlying the transition zone between the core of the Colorado Plateau and the rift may reflect processes resulting from the modern (Pliocene to present) regional stress field (oriented WNW-ESE), possibly heralding future extension across the Jemez lineament and transition zone.

Journal of Geophysical Research B: Solid Earth↗

USGS2013

Detection of tamarisk defoliation by the northern tamarisk beetle based on multitemporal Landsat 5 thematic mapper imagery.

The spread of tamarisk ( Tamarix spp., also known as saltcedar) is a significant ecological disturbance in western North America and has long been targeted for control, leading to the importation of the northern tamarisk beetle ( Diorhabda carinulata ) as a biological control agent. Following its initial release along the Colorado River near Moab, Utah in 2004, the beetle has successfully established and defoliated tamarisk across much of the upper Colorado River Basin. However, the spatial distribution and seasonal timing of defoliation are complex and difficult to quantify over large areas. To address this challenge, we tested and compared two remote sensing approaches to mapping tamarisk defoliation: Disturbance Index (DI) and a decision tree method called Random Forest (RF). Based on multitemporal Landsat 5 TM imagery for 2006-2010, changes in DI and defoliation probability from RF were calculated to detect tamarisk defoliation along the banks of Green, Colorado, Dolores and San Juan rivers within the Colorado Plateau area. Defoliation mapping accuracy was assessed based on field surveys partitioned into 10 km sections of river and on regions of interest created for continuous riparian vegetation. The DI method detected 3711 ha of defoliated area in 2007, 7350 ha in 2008, 10,457 ha in 2009 and 5898 ha in 2010. The RF method detected much smaller areas of defoliation but proved to have higher accuracy, as demonstrated by accuracy assessment and sensitivity analysis, with 784 ha in 2007, 960 ha in 2008, 934 ha in 2009, and 1008 ha in 2010. Results indicate that remote sensing approaches are likely to be useful for studying spatiotemporal patterns of tamarisk defoliation as the tamarisk leaf beetle spreads throughout the western United States.

Arizona, Colorado, New Mexico, Utah, Wyoming↗

USGS1985

Mineralization of breccia pipes in northern Arizona

The Paleozoic sedimentary rocks on the Colorado Plateau of northern Arizona are host to hundreds ofbreccia pipes. The uranium and copper deposits in these breccia pipes transgress formation boundaries from the Mississippian Redwall Limestone to the Triassic Chinle Formation. They are not classic breccia pipes in that there is no volcanic rock associated with them in time or space. They are the result of solution-collapse within the Redwall Limestone and stoping of the overlying strata. The karst development in the Redwall Limestone began in the Mississippian and apparently either continued to the Triassic or was at least once again active during that time. The mineralization apparently occurred shortly thereafter, sometime during the Mesozoic. Mining activity in breccia pipes of the Grand Canyon region began during the nineteenth century and continues today with the operation of the Hack I, II, and III mines, although the exploited commodity has changed from Cu to U. Although small in size, these pipes contain samples with up to 55 percent U 3 O 8 and can yield ore averaging between 0.30 and 0.60 percent U 3 O 8 .Mineralization at the surface commonly occurs within nodules and concretions associated with pyrite and goethite and along fractures, while the primary ore of the unoxidized zones is commonly within a comminuted sandstone matrix surrounding breccia fragments of overlying formations. The ore mineral is uraninite, although associated with it are sphalerite, galena, chalcopyrite, tennantite, millerite, siegenite, and/molybdenite. Some of the surface nodules are encrusted with malachite and are exceptionally enriched in Ag. Pyrite is abundant, and the organic carbon content of some rocks is high enough to suggest that it, along with the pyrite, may be a reductant for uranium. In contrast, it is possible, if uranium were transported as a bicarbonate or carbonate complex, that only a conduit of brecciated rock was necessary to release CO 2 , thus disrupting the equilibrium and allowing uraninite to precipitate. An extensive suite of elements is significantly enriched in the mineralized rock: Ag, As, Ba, Cd, Co, Cr, Cs, Cu, Hg, Mo, Ni, Pb, Sb, Se, Sr, U, V, Zn, and the rare earth elements. Of these, Cu, Pb, Zn, Ag, and particularly As appear to be the best geochemical indicators of mineralized pipes. At present the origin of the mineralizing fluids is not known. The lack of extensive silification within the breccia, along with the 80 degrees to 173 degrees C fluid inclusion-filling temperatures on sphalerite, dolomite, quartz, and calcite, suggests relatively low-temperature mineralizing fluids, although heated in excess of what would be expected from the normal geothermal gradient on the Colorado Plateau. With the exception of the U-mineralized rock, the mineral assemblage and geochemistry is similar to Mississippi Valley-type deposits.

Economic Geology↗

USGS1991

Channel evolution and hydrologic variations in the Colorado River basin: Factors influencing sediment and salt loads

Suspended-sediment and dissolved-solid (salt) loads decreased after the early 1940s in the Colorado Plateau portion of the Colorado River basin, although discharge of major rivers - the Colorado, Green and San Juan - did not change significantly. This decline followed a period of high sediment yield caused by arroyo cutting. Reduced sediment loads have previously been explained by a change in sediment sampling procedures or changes in climate, land-use and conservation practices. More recent work has revealed that both decreased sediment production and sediment storage in channels of tributary basins produced the decline of sediment and salt loads. Sediment production and sediment storage are important components of incised-channel evolution, which involves sequential channel deepening, widening and finally floodplain formation. Accordingly, the widespread arroyo incision of the late nineteenth century resulted initially in high sediment loads. Since then, loads have decreased as incised channels (arroyos) have stabilized and begun to aggrade. However, during the 1940s, a period of low peak discharges permitted vegetational colonization of the valley floors, which further reduced sediment loads and promoted channel stabilization. This explanation is supported by experimental studies and field observations. Both geomorphic and hydrologic factors contributed to sediment storage and decreased sediment and salt loads in the upper Colorado River basin.

Arizona, California, Colorado, Nevada, New Mexico,↗

USGS2012

Arizona/New Mexico Plateau Ecoregion: Chapter 26 in Status and trends of land change in the Western United States--1973 to 2000

Situated between ecoregions of distinctly different topographies and climates, the Arizona/New Mexico Plateau Ecoregion represents a large area of approximately 192,869 km 2 (74,467 mi 2 ) that stretches across northern Arizona, central and northwestern New Mexico, and parts of southwestern Colorado; in addition, a small part extends into southeastern Nevada (fig. 1) (Omernik, 1987; U.S. Environmental Protection Agency, 1997). Forested, mountainous terrain borders the ecoregion on the northeast (Southern Rockies Ecoregion) and southwest (Arizona/New Mexico Mountains Ecoregion). Warmer and drier climates exist to the south (Chihuahuan Deserts Ecoregion) and west (Mojave Basin and Range Ecoregion). The semiarid grasslands of the western Great Plains are to the east (Southwestern Tablelands Ecoregion), and the tablelands of the Colorado Plateau in Utah and western Colorado lie to the north (Colorado Plateaus Ecoregion). The Arizona/New Mexico Plateau Ecoregion occupies a significant portion of the southern half of the Colorado Plateau.

Arizona;Colorado;Nevada;New Mexico↗

USGS2010

Characterization of ten microsatellite loci in midget faded rattlesnake (Crotalus oreganus concolor)

Primers for 10 microsatellite loci were developed for midget faded rattlesnake ( Crotalus oreganus concolor ), a small bodied subspecies of the Western Rattlesnake, which is found in the Colorado Plateau of eastern Utah, western Colorado and southwestern Wyoming. In a screen of 23 individuals from the most northern portion of the subspecies range in southwestern Wyoming, the 10 loci were found to have levels of variability ranging from 4 to 11 alleles. No loci were found to be linked, although one locus revealed significant departures from Hardy–Weinberg equilibrium. These microsatellite loci will be applicable for population genetic analyses, which will ultimately aid in management efforts for this rare subspecies of rattlesnake.

Conservation Genetics Resources↗

USGS2004

Geochemical cycles in sediments deposited on the slopes of the Guaymas and Carmen Basins of the Gulf of California over the last 180 years

Sediments deposited on the slopes of the Guaymas and Carmen Basins in the central Gulf of California were recovered in two box cores. Q-mode factor analyses identified detrital-clastic, carbonate, and redox associations in the elemental composition of these sediments. The detrital-clastic fraction appears to contain two source components, a more mafic component presumably derived from the Sierra Madre Occidental along the west coast of Mexico, and a more felsic component most likely derived from sedimentary rocks (mostly sandstones) of the Colorado Plateau and delivered by the Colorado River. The sediments also contain significant siliceous biogenic components and minor calcareous biogenic components, but those components were not quantified in this study. Redox associations were identified in both cores based on relatively high concentrations of molybdenum, which is indicative of deposition under conditions of sulfate reduction. Decreases in concentrations of molybdenum in younger sediments suggest that the bottom waters of the Gulf have became more oxygenated over the last 100 years. Many geochemical components in both box cores exhibit distinct cyclicity with periodicities of 10-20 years. The most striking are 20-year cycles in the more mafic components (e.g., titanium), particularly in sediments deposited during the 19th century. In that century, the titanium cycles are in very good agreement with warm phases of the Pacific Decadal Oscillation, implying that at times of greater influx of titanium-rich volcanic debris, there were more El Nin??os and higher winter precipitation. The cycles are interpreted as due to greater and lesser riverine influx of volcanic rock debris from the Sierra Madre. There is also spectral evidence for periodicities of 4-8 and 8-16 years, suggesting that the delivery of detrital-clastic material is responding to some multiannual (ENSO?) forcing.

Quaternary Science Reviews↗

USGS2019

In situ distributions of magnetic susceptibility in some igneous rocks

Measurements of in-situ magnetic susceptibility were compiled from mainly Precambrian crystalline basement rocks beneath the Colorado Plateau and ranges in Arizona, Colorado, and New Mexico. The susceptibility meter used samples about 33 cubic centimeters of rock and measures variations in the modal distribution of magnetic minerals that form a minor component volumetrically in these coarsely crystalline granitic to granodioritic rocks. Recent measurements include 50-150 measurements on each outcrop and show that the distribution of magnetic susceptibilities is highly variable, multimodal and strongly non-Gaussian so that a mean value has little significance. Rock bodies with the most multimodal distributions generally have complex tectonic histories including metamorphism and multiple tectonic events. Variations between outcrops within the same rock body are large; however, where distributions overlap, measurements appear to fill gaps within modal peaks. Histograms of measurements are a better representation of the magnetic susceptibility distribution for a given rock body than mean and standard deviation. The best effective magnetic susceptibility estimate for an outcrop can be obtained by computing themagnetic force of the measurements 3-5 m above the outcrop and finding the constant susceptibility that gives an equal integral of the force. The multifractal distribution of the minor minerals in the rocks explains the observed multimodal distributions of magnetic susceptibility at millimeter to meter scales.

Arizona, Colorado, New Mexico↗

USGS2017

The U.S. Geological Survey Flagstaff Science Campus—Providing expertise on planetary science, ecology, water resources, geologic processes, and human interactions with the Earth

T he U.S. Geological Survey’s Flagstaff Science Campus is focused on interdisciplinary study of the Earth and solar system, and has the scientific expertise to detect early environmental changes and provide strategies to minimize possible adverse effects on humanity. The Flagstaff Science Campus (FSC) is located in Flagstaff, Arizona, which is situated in the northern part of the State, home to a wide variety of landscapes and natural resources, including (1) young volcanoes in the San Francisco Volcanic Field, (2) the seven ecological life zones of the San Francisco Peaks, (3) the extensive geologic record of the Colorado Plateau and Grand Canyon, (4) the Colorado River and its perennial, ephemeral, and intermittent tributaries, and (5) a multitude of canyons, mountains, arroyos, and plains. More than 200 scientists, technicians, and support staff provide research, monitoring, and technical advancements in planetary geology and mapping, biology and ecology, Earth-based geology, hydrology, and changing climate and landscapes. Scientists at the FSC work in collaboration with multiple State, Federal, Tribal, municipal, and academic partners to address regional, national, and global environmental issues, and provide scientific outreach to the general public.

Arizona↗

USGS1975

Lithology and chemical analyses of core and cuttings from USGS drill hole near Gold Acres, Lander County, Nevada

Upper Paleozoic to Mesozoic eolian blanket sandstones of the Colorado Plateau and the Rocky Mountains of Colorado and southern Wyoming are texturally complex. As petroleum reservoirs they commonly have poor performance histories. They contain the sediments of a depositional system comprised of three closely associated depositional subenvironments: dune, interdune, and extradune. Sediments of each subenvironment have different textural properties which resulted from different depositional processes. Dune sediments are usually more porous and permeable than interdune or extradune sediments and may be better quality reservoirs than interdune or extradune sediments. Interdune sediments are here restricted to those nondune sediments deposited in the relatively flat areas between dunes. Extradune sediments (a new term) include all deposits adjacent to a dune field and are mainly subaqueous deposits. Dune sediments may be enveloped by extradune sediments as the depositional system evolves resulting in a texturally inhomogeneous reservoir having poor fluid migration properties. This model of textural inhomogeneity in eolian blanket sandstones. was applied to the Weber (Tensleep) Sandstone in Brady, Wertz, and Lost Soldier fields, Sweetwater County, Wyoming. Data were obtained from both outcrop and subsurface and included environmental interpretation, textural analysis, and plotting of the distribution of depositional subenvironments. As predicted from the model, the texture of dune sediments in Brady field differed markedly from interdune and extradune sediments. The predicted geometric distribution of subenvironments was confirmed in Lost Soldier and Wertz fields. However, secondary cementation and fracturing there has obscured the original porosity and permeability contrasts. The porosity and permeability distribution, a characteristic depending partly on depositional processes, could impede fluid migration in the reservoir and significantly reduce recovery of hydrocarbons.

Open-File Report↗

USGS1990

Microphytic crusts: 'topsoil' of the desert

Deserts throughout the world are the home of microphytic, or cryptogamic, crusts. These crusts are dominated by cyanobacteria, previously called blue-green algae, and also include lichens, mosses, green algae, microfungi and bacteria. They are critical components of desert ecosystems, significantly modifying the surfaces on which they occur. In the cold deserts of the Colorado Plateau (including parts of Utah, Arizona, Colorado, and New Mexico), these crusts are extraordinarily well-developed, and may represent 70-80% of the living ground cover.

Permaculture Drylands Journal↗

USGS2018

Population dynamics of the northern tamarisk beetle ( Diorhabda carinulata ) in the Colorado River Basin

Throughout the Southwestern United States, riparian systems contain narrow belts of vegetation along streams and rivers. Although only a small percentage of the total land cover, this ecosystem is important for maintaining high species diversity and population densities of birds. Anthropogenic changes to Western riverine systems have enhanced their susceptibility to invasion by introduced plant species, in particular, ornamental plants from the genus Tamarix (or saltcedar), which can establish itself in dry, salty conditions and spread rapidly. Recently, the central Asian saltcedar leaf beetle ( Diorhabda carinulata ) was released as a biocontrol for tamarisk. Since its release on the Colorado Plateau, tamarisk beetle populations in Nevada, Utah, Colorado, and Wyoming have widely expanded, leading to widespread tamarisk defoliation, and concerns from land managers regarding the consequences of the environmental impact. Defoliation can also negatively impact avian communities in the short term by decreasing insect abundance and nesting success, owing to increased solar radiation or loss of camouflage. This report details two studies that examine the spread of the introduced tamarisk beetle over parts of the Southwestern United States. The first chapter documents plant phenology and beetle abundance and movement along the Dolores and San Juan Rivers, two major tributaries of the Colorado River. This study demonstrates that D. carinulata population-movement patterns can be highly influenced by the availability of beetle food resources and that local beetle “boom and bust” events are common. The second study demonstrates that the extent and timing of tamarisk defoliation are predictable on the basis of (1) abiotic cues for D. carinulata activity, (2) spatial distributions and abundances of D. carinulata across a site, and (3) movement of D. carinulata as a result of available tamarisk foliage. A significant positive correlation exists between the spatial distributions of D. carinulata populations in the fall and those of the first generation of larvae in the following spring, suggesting that the extent of tamarisk defoliation as a result of abundant larval populations is predictable. The results of these two studies will enable conservationists to better understand the variable timing of tamarisk defoliation events across a landscape and provide a template to forecast tamarisk defoliation levels and rates in areas that have yet to be colonized by D. carinulata .

Colorado, Utah↗

USGS1995

Water-quality assessment of the Rio Grande Valley study unit, Colorado, New Mexico, and Texas -- Analysis of selected nutrient, suspended-sediment, and pesticide data

This report contains a summary of data compiled from sources throughout the Rio Grande Valley study unit of the National Water-Quality Assessment program. Information presented includes the sources and types of water-quality data available, the utility of water-quality data for statistical analysis, and a description of recent water-quality conditions and trends and their relation to natural and human factors. Water-quality data are limited to concentrations of selected nutrient species in surface water and ground water, concentrations of suspended sediment and suspended solids in surface water, and pesticides in surface water, ground water, and biota. The Rio Grande Valley study unit includes about 45,900 square miles in Colorado, New Mexico, and Texas upstream from the streamflow-monitoring station Rio Grande at El Paso, Texas. The area also includes the San Luis Closed Basin and the surface-water closed basins east of the Continental Divide and north of the United States-Mexico international border. The Rio Grande drains about 29,300 square miles in these States; the remainder of the study unit area is in closed basins. Concentrations of all nutrients found in surface-water samples collected from the Rio Grande, with the exception of phosphorus, generally remained nearly constant from the northernmost station in the study unit to Rio Grande near Isleta, where concentrations were larger by an order of magnitude. Total nitrogen and total phosphorus loads increased downstream between Lobatos, Colorado, and Albuquerque, New Mexico. Nutrient concentrations remained elevated with slight variations until downstream from Elephant Butte Reservoir, where nutrient concentrations were lower. Nutrient concentrations then increased downstream from the reservoir, as evidenced by elevated concentrations at Rio Grande at El Paso, Texas. Suspended-sediment concentrations were similar at stations upstream from Otowi Bridge near San Ildefonso, New Mexico. The concentration and estimated load were nearly two orders of magnitude larger at this station relative to upstream stations. Cochiti Lake allows suspended sediment to settle, thus the resulting concentration is substantially lower downstream from the reservoir. Downstream from Cochiti Lake, concentrations again increased due to inflow from tributaries, other ephemeral streams and arroyos, and agricultural and urban areas. Two ephemeral tributaries (Rio Puerco and Rio Salado, which are south of Albuquerque) contribute substantial amounts of suspended sediment to the Rio Grande. Suspended-sediment concentrations in the Rio Grande just downstream from Elephant Butte Dam decreased by nearly three orders of magnitude due to settling in the reservoir. Concentrations then increased due to agricultural and urban impacts downstream from the reservoir. Nutrients in ground water in the study unit do not appear to be a widespread problem. However, localized areas that have elevated nitrate concentrations have been documented. The largest median nitrate concentration was found in water from wells located in the Basin and Range-mountains-urban data stratum (3.0 milligrams per liter) and the smallest median nitrate concentration was found in water from wells located in the Southern Rocky Mountainsmountains-forest data stratum (0.08 milligram per liter). Few (3 percent) nitrate concentrations in water from wells in all data strata were greater than 10 milligrams per liter, and most (82 percent) were less than 2 milligrams per liter. Comparison of nitrate concentrations in water from wells located in specific land-use settings across all hydrogeologic settings, with the exception of the Colorado Plateau, indicated that the largest median nitrate concentration was associated with rangeland land use and that larger nitrate concentrations were found in water from shallow wells. Water from wells located in areas of rangeland land use consistently had larger median nutrient concentrations than water from wells in areas of other land uses. The largest median ammonia concentration was in water from wells located in the Colorado Plateau-San Juan Basin-rangeland data stratum (0.27 milligram per liter). Most median ammonia concentrations were less than 0.03 milligram per liter, indicating that elevated ammonia concentrations are not a major issue in the study unit. The largest median orthophosphate concentration was found in water from wells located in the Southern Rocky Mountains-mountains-forest data stratum (0.15 milligram per liter) and the smallest was found in water from wells located in the Basin and Range-mountains-urban data stratum (0.02 milligram per liter). Most orthophosphate concentrations (85 percent) sampled were less than 0.2 milligram per liter, indicating that elevated orthophosphate concentrations are not a major issue in the study unit. Pesticide analyses were available for only 38 ground-water sampling sites in the Rio Grande Valley study unit. Diazinon, at a concentration of 0.01 microgram per liter, was the only pesticide detected and it was detected at only one site. More study is needed to determine if pesticides are affecting ground-water quality in the Rio Grande Valley study unit. Surface-water biological pesticide data were inadequate for in-depth analysis. The primary sources of data were the U.S. Fish and Wildlife Service and the U.S. Geological Survey. In the U.S. Fish and Wildlife Service study p,p'-DDE, a degradation product of DDT, was detected most frequently; highest concentrations were found at Stahman Farms in carp (6.3 micrograms per gram wet-weight) and at Hatch in Western kingbird (5.1 micrograms per gram wet-weight). In the U.S. Geological Survey study of Bosque del Apache National Wildlife Refuge no detectable organochlorine concentrations were found in plants, but detectable levels of p,p'-DDE were found in coot and carp, with a maximum concentration of 0.12 microgram per gram wet-weight found in coot.

Colorado, New Mexico, Texas↗

USGS2005

Episodic incision of the Colorado River in Glen Canyon, Utah

Incision rates of the Colorado River are integral to understanding the development of the Colorado Plateau. Here we calculate episodic incision rates of the Colorado River based on absolute ages of two levels of Quaternary deposits adjacent to Glen Canyon, Utah, along the north flank of Navajo Mountain. Minimum surface ages are determined by a combination of cosmogenic radionuclide surface exposure ages, uranium series and soil-development formation times. Bedrock incision rates of the Colorado River between c. 500 ka and c. 250 ka, and c. 250 ka to present are c. 0??4 m ka-1 and c. 0??7 m ka-1, respectively. These rates are more than double the rates reported in the Grand Canyon, suggesting that the Colorado River above Lees Ferry is out of equilibrium with the lower section of the river. We also determine incision rates of two tributaries to the Colorado River. Oak Creek and Bridge Creek flow off Navajo Mountain into Glen Canyon from the southeast. Oak Creek and Bridge Creek both have incision rates of c. 0??6 m ka-1 over the past c. 100 ka at points about 9 km away from the main stem of the Colorado River. Copyright ?? 2005 John Wiley & Sons, Ltd.

Earth Surface Processes and Landforms↗