Upper Colorado Research Articles

Upper Colorado River Basin 20th century droughts under 21st century warming: Plausible scenarios for the future

This study builds on a collaboration with a water resource management community of practice in the Upper Colorado River Basin to develop scenarios of future drought and assess impacts on water supply reliability. Water managers are concerned with the impacts of warming on water year streamflow, but uncertainties in projections of climate make the application of these projections to planning a challenge. Instead, water managers considered a plausible scenario for future drought to be historical droughts to which warming is added. We used a simple statistical model of water year streamflow with temperatures increased by 1 °C to 4 °C, and then examined reductions in flow and runoff efficiency (RE) with each degree of warming for the six droughts defined in the observed streamflow record. In order to place these results into a management context, we employed an existing framework for system reliability, and in particular, a vulnerability assessment for water delivery metrics. Using modeled streamflow resulting from 1 °C to 4 °C warming, we found vulnerable condition thresholds for the two water delivery metrics assessed, Upper Basin Shortage and Lees Ferry Deficit, were crossed relatively infrequently at +1 °C, but with a substantially increased frequency under additional warming. Results are more relevant to resource management because the impacts of warming on Upper Colorado River streamflow were assessed in the context of management metrics and vulnerability thresholds, in collaboration with members of the water management community of practice.

Riparian Plant Communities Remain Stable in Response to a Second Cycle of Tamarix Biocontrol Defoliation

Reduced abundance of non-native Tamarix shrubs in western U.S. riparian systems following biological control by a defoliating beetle has led to concerns that replacement plant communities could be dominated by other invasive species and/or not provide some of the ecosystem services that Tamarix was providing. In previous studies, Tamarix decline following biocontrol was accompanied by small increases in native and non-native herbaceous species, with variable responses of woody vegetation. However, none of these studies spanned periods longer than a decade since beetle release. This is an important caveat, given the cyclical nature of plant-herbivore interactions and potential lags in vegetation recovery. We report plant community response to an eight-year-long second cycle of Tamarix defoliation-refoliation in two reaches of the upper Colorado River in eastern Utah, 11–13 years after beetle arrival. Tamarix cover across sites initially declined an average of ca. 50% in response to the beetle, but then recovered. Changes in the associated plant community were small but supported common management goals, including a 47% average increase in cover of a native shrub (Salix exigua), and no secondary invasions by other non-native plants. We suggest that the effectiveness of biocontrol programs must be assessed case-by-case, and on a long-term basis.

Deriving representative reservoir operation rules using a hidden Markov-decision tree model

Reservoirs have been widely used to regulate streamflow to meet both human and natural water requirements. This study applies a hidden Markov-decision tree (HM-DT) model to derive representative reservoir operation modules under various operation conditions (i.e., inflow, storage, as well as unknown factors) and their transitions (the dynamic change of operation rules) that reflect the impacts of seasonality, long-term non-stationarity, and extreme events on reservoir operation. The representative operation modules can be applied to reservoirs in the same region that are not observed; the capability for simulating dynamic operation behaviors improves the predictive accuracy as compared to regular decision trees. Using a number of reservoirs located in the same region for training, the HM-DT model can derive a limited number of representative operation modules in the form of decision trees (DT), and the transitions between different operation schemes in response to changing operation conditions. The application of the HM-DT model is demonstrated through a case study of the Upper Colorado River basin, where eight representative operation modules are determined for 50 reservoirs located in the region, and the modules are validated with 11 reservoirs in the same region. The eight operation modules are classified into three types (i.e. nearly constant release, release as a piece-wise function of inflow, and release almost identical to inflow). The identified operation modules and the transition patterns between operation modules can be used to better understand real-world operation behaviors, improve future operations, and build realistic reservoir operation components in basin-scale hydrological models.

Ranchers Adapting to Climate Variability in the Upper Colorado River Basin, Utah

In the Upper Colorado River Basin, agriculture is a major contributor to Utah’s economy, which may be stressed due to the changing climate. In this study, two data-mining techniques and interview data are used to explore how climate variability affects agricultural production and the way the farmers have been adapting their practices to these changes. In the first part of the study, we used multilinear regression and random forest regression to understand the relationship between climate and agricultural production using temperature, precipitation, water availability, hay production, and cattle herd size. The quantitative results showed weak relations among variables. In the second part of the study, we interviewed ranchers to fill the gaps in the quantitative analysis. Over the 35 years (1981–2015), the quantitative analysis shows that temperature has affected cattle and hay production more than precipitation. Among non-climatic variables, resource availability and commodity prices are the most important factors that influence year-to-year production. Farmers are well-aware of these effects and have adapted accordingly. They have changed irrigation practices, cropping patterns, and are experimenting to produce a hybrid species of cattle, that are resilient to a hotter temperature and can use a wider variety of forage.

Modeling interdependent water uses at the regional scale to engage stakeholders and enhance resilience in Central Arizona

As cities and agricultural areas face water challenges associated with climate change, it is important to develop a better understanding of how human and natural systems will respond at the scales at which those changes will occur. This requires analytical tools to systematically explore regional contexts where multiple interdependent water, agricultural, and urban infrastructures interact. Toward this end, we develop and analyze a stylized model of a regional-scale system in the Southwestern U.S. The system is comprised of the Phoenix and Tuscon metropolitan areas and surrounding agricultural districts within the Central Arizona Project service domain and the water delivery infrastructure that connects these areas to the Colorado River basin. We use the model to analyze the impacts of changes in runoff in the upper Colorado River basin and the Salt, Verde, and Agua Fria basins on water supplies in the Central Arizona Project service area. Specifically, we explore how conceptualizing the Phoenix and Tuscon metropolitan areas and surrounding agricultural districts as an integrated system of urban, agricultural, and conjunctive ground and surface water management infrastructures can lead to strategies to meet water demand while maintaining groundwater neutrality.

Movement ecology of imperilled fish in a novel ecosystem: River‐reservoir movements by razorback sucker and translocations to aid conservation

Abstract Reservoirs and associated river fragments are novel ecosystems not experienced by fishes in their evolutionary history, yet they are now commonplace across the globe. Understanding how fishes use these novel habitats is vital to conservation efforts in contemporary riverscapes. Movement patterns of the endangered razorback sucker (Xyrauchen texanus) synthesized from tagging efforts in the upper Colorado River basin, USA, illustrate the applications of tagging technology and data sharing by multiple agencies to better understand the spatial ecology of large river fishes. Tagging studies between 2014 and 2018 in Lake Powell and its two main tributary rivers, the Colorado (unfragmented) and San Juan (waterfall‐fragmented), were used to quantify movement of razorback sucker within this river–reservoir habitat complex. In addition, facilitated translocations of fish upstream of a waterfall barrier in the San Juan River were assessed in 2016–2017. Extensive movement of fish occurred within and across river and reservoir habitats. Of 722 fish captured in the Colorado River arm of Lake Powell, 36% of re‐encounters occurred upstream in the Colorado or Green rivers, or fish dispersed through the reservoir and were detected in the San Juan River arm. Fourteen fish moved more than 600 km. In the San Juan arm of the reservoir, 29% and 20% of fish in 2017 and 2018, respectively, had moved ~30–40 km upstream below the waterfall in the San Juan River within a year. In 2016–2017, 303 fish were translocated upstream of the waterfall into the San Juan River, but 80% were re‐encountered downstream of the waterfall within a year. Long‐distance movements by razorback sucker were common within and among rivers and reservoirs illustrating how large river fish, in general, might maintain population connectivity in highly altered ecosystems.

Diverse Thaumarchaeota Dominate Subsurface Ammonia-oxidizing Communities in Semi-arid Floodplains in the Western United States.

Subsurface microbial communities mediate biogeochemical transformations that drive both local and ecosystem-level cycling of essential elements, including nitrogen. However, their study has been largely limited to the deep ocean, terrestrial mines, caves, and topsoils (<30cm). Here, we present regional insights into the microbial ecology of aerobic ammonia oxidation within the terrestrial subsurface of five semi-arid riparian sites spanning a 900-km N-S transect. We sampled sediments, profiled communities to depths of ≤ 10m, and compared them to reveal trends regionally within and surrounding the Upper Colorado River Basin (CRB). The diversity and abundance of ammonia-oxidizing microbial communities were evaluated in the context of subsurface geochemistry by applying a combination of amoA (encoding ammonia monooxygenase subunit A) gene sequencing, quantitative PCR, and geochemical techniques. Analysis of 898 amoA sequences from ammonia-oxidizing archaea (AOA) and bacteria (AOB) revealed extensive ecosystem-scale diversity, including archaeal amoA sequences from four of the five major AOA lineages currently found worldwide as well as distinct AOA ecotypes associated with naturally reduced zones (NRZs) and hydrogeochemical zones (unsaturated, capillary fringe, and saturated). Overall, AOA outnumber AOB by 2- to 5000-fold over this regional scale, suggesting that AOA may play a prominent biogeochemical role in nitrification within terrestrial subsurface sediments.

Divide mobility controls knickpoint migration on the Roan Plateau (Colorado, USA)

Abstract Knickpoints in longitudinal river profiles are proxies for the climatic and tectonic history of active mountains. The analysis of river profiles commonly relies on the assumption that drainage network configurations are stable. Here, we show that this assumption must be made cautiously if changes in contributing area are fast relative to knickpoint migration rates. We studied the Parachute Creek basin in the Roan Plateau, Colorado, United States, where knickpoint retreat occurs in horizontally uniform lithology so that drainage area is the sole governing variable. In this basin, we identified an anomalous catchment in the degree to which a stream power–based model predicted knickpoint locations. The catchment is experiencing area loss as the plateau edge is eroded by cliff migration in proximity to the Colorado River. Model predictions improve if the plateau edge is assumed to have migrated over the time scale of knickpoint retreat. Finally, a Lagrangian model of knickpoint migration enabled us to study the kinematic links between drainage area loss and knickpoint migration and offered constraints on the temporal aspects of area loss. Modeled onset and amount of area loss are consistent with cliff retreat rates along the margin of the Roan Plateau inferred from the incisional history of the upper Colorado River.

Simulating Groundwater-Streamflow Connections in the Upper Colorado River Basin.

In mountain, snow driven catchments, snowmelt is supposed to be the primary contribution to river streamflows during spring. In these catchments the contribution of groundwater is not well documented because of the difficulty to monitor groundwater in such complex environment with deep aquifers. In this study we use an integrated hydrologic model to conduct numerical experiments that help quantify the effect of lateral groundwater flow on total annual and peak streamflow in predevelopment conditions. Our simulations focus on the Upper Colorado River Basin (UCRB; 2.8 × 105 km2 ) a well-documented mountain catchment for which both streamflow and water table measurements are available for several important sub-basins. For the simulated water year, our results suggest an increase in peak flow of up to 57% when lateral groundwater flow processes are included-an unexpected result for flood conditions generally assumed independent of groundwater. Additionally, inclusion of lateral groundwater flow moderately improved the model match to observations. The correlation coefficient for mean annual flows improved from 0.84 for the no lateral groundwater flow simulation to 0.98 for the lateral groundwater flow one. Spatially we see more pronounced differences between lateral and no lateral groundwater flow cases in areas of the domain with steeper topography. We also found distinct differences in the magnitude and spatial distribution of streamflow changes with and without lateral groundwater flow between Upper Colorado River Sub-basins. A sensitivity test that scaled hydraulic conductivity over two orders of magnitude was conducted for the lateral groundwater flow simulations. These results show that the impact of lateral groundwater flow is as large or larger than an order of magnitude change in hydraulic conductivity. While our results focus on the UCRB, we feel that these simulations have relevance to other headwaters systems worldwide.

Holocene extreme paleofloods and their climatological context, Upper Colorado River Basin, USA

Given its singular importance for water resources in the southwestern USA, the Upper Colorado River Basin (UCRB) is remarkable for the paucity of its conventional hydrological record of extreme flooding. Short-term record-based flood frequency analyses lead to very great aleatory uncertainties about infrequent extreme flood events and their climate-driven causal associations. This study uses paleoflood hydrology to examine a small portion of the underutilized, but very extensive natural record of Holocene extreme floods in the UCRB. We perform a meta-analysis of 82 extreme paleofloods from 18 slack water deposit sites in the UCRB to show linkages between Holocene climate patterns and extreme floods. The analysis demonstrates several clusters of extreme flood activity: 8040–7960, 4400–4300, 3600–3460, 2900–2740, 2390–1980, 1810–720, and 600–0 years BP. The extreme paleofloods were found to occur during both dry and wet periods in the paleoclimate record. When compared with independent paleoclimatic records across the Rocky Mountains and the southwestern USA, the observed temporal clustering pattern of UCRB extreme paleofloods shows associations with periods of abruptly intensified North Pacific-derived storms connected with enhanced variability of El Niño. This approach demonstrates the value of creating paleohydrological databases and comparing them with hydro-climatic proxies in order to identify natural patterns and to discover possible linkages to fundamental processes such as changes in climate.

Projected Changes to Extreme Runoff and Precipitation Events From a Downscaled Simulation Over the Western United States

Extreme precipitation and runoff events, which often impact natural and social systems more than mean changes, generally occur over regional scales. Future climate projections can be used to estimate how the hydrologic cycle may change, but the coarse resolution of global climate models (>1°) makes it difficult to evaluate regional changes, such as over a single watershed. To estimate changes in hydroclimatic variables at finer spatial resolutions, we dynamically downscale the Community Earth System Model (CESM) with the Weather Research and Forecasting (WRF) regional climate model over the western United States at 9 km spatial resolution. By running WRF at a higher spatial resolution, we estimate future climate conditions, including 99% and 99.9% event magnitude, over seventeen watersheds: the Columbia, Lower Colorado, Upper Colorado, the Upper Missouri/Yellowstone, and twelve basins draining the western slope of the Sierra Nevada in California. Over each basin, we compare a historical period (1996-2005) with mid-century (2041-2050) and end-century (2091-2100). From the WRF/CESM simulations, most basins are projected to have earlier peaks in springtime streamflow. The Columbia and the Lower Colorado watersheds are both expected to experience more extreme wet days, with the 99th percentile of daily precipitation estimated to increase by over 10%. For the Upper Colorado, however, the 99th percentile of daily runoff is projected to decrease by over 30%. Basins in the northern and central Sierra Nevada are projected to have substantial increases in extreme runoff, with doubling of high flow event magnitude possible for some basins. By end-century, the contribution of high-magnitude runoff (>90th percentile) to total runoff is projected to increase from 46% to 56%, when averaged across all twelve Sierra Nevada basins. Though only one realization from a single global climate model, the downscaled simulation presented here shows interesting results regarding how extreme events may change; these results can be tested by downscaling other global models with WRF to create an ensemble of dynamically downscaled future projections.

Bias Correction of Paleoclimatic Reconstructions: A New Look at 1,200+ Years of Upper Colorado River Flow

AbstractBias correction, while widely used with climate‐model output, is not typically applied to paleoclimatic reconstructions. While many reconstruction models have low average error, they still may contain bias, especially in the tails of distributions. Bias correction, used cautiously, can be a valuable procedure that alters interpretations of past events. Analyzing the iconic tree‐ring reconstruction of Upper Colorado River flow, we find that its probability distribution is markedly different from that of observed flow. Using quantile mapping to bias correct the reconstruction, we analyze the full reconstructed record and two events in particular: the 1100s megadrought and the early 1600s pluvial. Overall, bias correction made the 1100s megadrought, the largest in the 1,200+year record, even more extreme. After bias correction, the early 1600s pluvial marginally exceeds the early twentieth century pluvial in magnitude but not in duration. Overall, bias correction should be considered whenever paleoclimatic reconstructions are compared directly to observations.

Mercury and selenium concentrations in fishes of the Upper Colorado River Basin, southwestern United States: A retrospective assessment.

Mercury (Hg) and selenium (Se) are contaminants of concern for fish in the Upper Colorado River Basin (UCRB). We explored Hg and Se in fish tissues (2,324 individuals) collected over 50 years (1962–2011) from the UCRB. Samples include native and non-native fish collected from lotic waterbodies spanning 7 major tributaries to the Colorado River. There was little variation of total mercury (THg) in fish assemblages basin-wide and only 13% (272/1959) of individual fish samples exceeded the fish health benchmark (0.27 μg THg/g ww). Most THg exceedances were observed in the White-Yampa tributary whereas the San Juan had the lowest mean THg concentration. Risks associated with THg are species specific with exceedances dominated by Colorado Pikeminnow (mean = 0.38 and standard error ± 0.08 μg THg/g ww) and Roundtail Chub (0.24 ± 0.06 μg THg/g ww). For Se, 48% (827/1720) of all individuals exceeded the fish health benchmark (5.1 μg Se/g dw). The Gunnison river had the most individual exceedances of the Se benchmark (74%) whereas the Dirty Devil had the fewest. We identified that species of management concern accumulate THg and Se to levels above risk thresholds and that fishes of the White-Yampa (THg) and Gunnison (Se) rivers are at the greatest risk in the UCRB.

Evaluating spatiotemporal variation in water chemistry of the upper Colorado River using longitudinal profiling

AbstractStream water chemistry is traditionally measured as variation over time at fixed sites, with sparse sites providing a crude understanding of spatial heterogeneity. An alternative Lagrangian reference frame measures changes with respect to both space and time as water travels through a network. Here, we collected sensor‐based measurements of water chemistry at high spatial resolution along nearly 500 km of the Upper Colorado River. Our objective was to understand sources of spatiotemporal heterogeneity across different solutes and determine whether longitudinal change manifests as smooth gradients as suggested by the River Continuum Concept (RCC) or as abrupt changes as suggested by the Serial Discontinuity Concept (SDC). Our results demonstrate that Lagrangian sampling integrates spatiotemporal variation, and profiles reflect processes that vary in both space and time and over different scales. Over each day of sampling, water temperature (T) and dissolved oxygen (DO) varied strongly in response to diel solar cycles, with most of the variation driven by sampling time rather than sampling location. Equilibration of T and DO with the atmosphere limited small scale spatial heterogeneity, with variation at the entire profile scale driven by regional climate gradients. As such, T and DO profiles more closely approximated the smooth gradients of the RCC (though including temporal sampling artefacts). Conversely, variation in specific conductance and nitrate (NO3‐N) was largely driven by spatial patterns of lateral inflows such as tributaries and groundwater. This resulted in discrete shifts in the profiles at or downstream of discontinuities, appearing as the profiles expected with the SDC. The concatenation of spatiotemporal variation that produces observed Lagrangian profiles presents interpretive challenges but also augments our understanding of where, how, and critically why water chemistry changes in time and space as it moves through river networks.

Forecast-informed hydropower optimization at long and short-time scales for a multiple dam network

This study presents a scheme for co-optimizing the long-term (seasonal) reservoir operating objectives with the short-term (daily) objectives for multi-dam networks to maximize hydropower generation. Long-term optimal reservoir storage provides temporal space to optimize operation of the dams at short-term based on forecasted reservoir inflow. This study asks if there is an added benefit of co-optimization of operations at long- and short-term scales for hydropower generation. The multi-objective optimization problem at both the temporal scales was simultaneously solved considering Pareto optimality between conflicting objectives. Constraints pertaining to flood control, dam safety, and environmental flow were imposed. The proposed scheme was implemented over a network of Blue Mesa, Morrow Point, and Crystal dams in the Upper Colorado Basin. Ensemble forecast forcings from publicly available numerical weather prediction and climate models were used as inputs for the daily and monthly scale inflow forecasts. The results show improvements of 41%, 27%, and 15% in hydropower generation using the co-optimization during wet, moderate, and dry years, respectively, against a benchmark that neglects inflow forecasts. This study demonstrates added benefit of co-optimizing the operations for hydropower generation based on short- and long-term forecasted reservoir inflow. Given that most dams today operate as a network in a river basin, we recommend moving away from single dam and single time scale optimization to a multiple-dam with long- and short-term scale co-optimization-based operations to make renewable energy generation more efficient.

The role of baseflow in dissolved solids delivery to streams in the Upper Colorado River Basin

An error has been identified in Tables 2 and 3 of the article “The role of baseflow in dissolved solids delivery to streams in the Upper Colorado River Basin” (DOI: 10.1002/hyp.11390) (Rumsey, Miller, Schwarz, Hirsch, and Susong (2017), published in Wiley Online Library on 4 December 2017 and in Hydrological Processes, 31, 4705–4718. In Table 2, values in the column labeled ‘Mean annual baseflow DS load, 1986 (mT)’ have been corrected. In Table 3, values in the column labeled ‘Mean annual baseflow DS load, 1990 (mT)’ and ‘Mean annual baseflow DS load, 2000 (mT)’ have been corrected. Corrected tables are shown below.

The Risk of Curtailment under the Colorado River Compact

Water supply in the Colorado River could drop so far in the next decade that the ability of the Upper Colorado River Basin states – Colorado, Utah, Wyoming, and New Mexico – to meet their legal obligations to downstream users in Nevada, Arizona, California, and Mexico would be in grave jeopardy. Legal institutions designed nearly a century ago are inadequate to address the significant risk of shortfall combined with uncertainty about whose water supplies would be cut, and by how much. This report indicates that declines in the Colorado River’s flow could force water curtailments in coming decades, posing a credible risk to Colorado communities and requiring serious consideration of insurance protection like demand management.

Paleoflood hydrology on the lower Green River, upper Colorado River Basin, USA: An example of a naturalist approach to flood-risk analysis

Through a comprehensive paleoflood hydrological investigation we document natural evidence for at least 27 high-magnitude paleofloods at six sites on the Lower Green River, Utah. Hydraulic analysis, using the Sedimentation and River Hydraulic-2D model (SRH-2D), shows that the responsible peak paleoflood discharges ranged between 500 and 7500 m3/s. At least 14 of these paleoflood discharge peaks exceed a level twice that of the maximum systematic record of gauged flows: 1929 m3/s. Geochronological analyses, employing optically stimulated luminescence (OSL) and radiocarbon dating techniques, demonstrate that these 14 largest paleoflood peaks occurred during the past 700 years. Integration of the paleoflood data into flood frequency analyses (FFA) reveals considerably higher values for the upper tails of the flood distribution than does a FFA based solely on the systematic gauged record, indicating that extreme floods are larger and more frequent than implied by the relatively short gauged record. Through examination of three approaches to extreme flood estimation – conventional FFA, probable maximum flood estimation (PMF), and paleoflood hydrology (PFH) – we show the significance of the natural evidence for advancing scientific understanding of extreme floods that naturally occur in the Colorado River system. We argue that this kind of scientific understanding is absolutely essential for achieving a credible evaluation of extreme flood risk in a watershed of immense importance to economic prosperity of the southwestern U.S.

Causes for the Century-Long Decline in Colorado River Flow

Abstract Upper Colorado River basin streamflow has declined by roughly 20% over the last century of the instrumental period, based on estimates of naturalized flow above Lees Ferry. Here we assess factors causing the decline and evaluate the premise that rising surface temperatures have been mostly responsible. We use an event attribution framework involving parallel sets of global model experiments with and without climate change drivers. We demonstrate that climate change forcing has acted to reduce Upper Colorado River basin streamflow during this period by about 10% (with uncertainty range of 6%–14% reductions). The magnitude of the observed flow decline is found to be inconsistent with natural variability alone, and approximately one-half of the observed flow decline is judged to have resulted from long-term climate change. Each of three different global models used herein indicates that climate change forcing during the last century has acted to increase surface temperature (~+1.2°C) and decrease precipitation (~−3%). Using large ensemble methods, we diagnose the separate effects of temperature and precipitation changes on Upper Colorado River streamflow. Precipitation change is found to be the most consequential factor owing to its amplified impact on flow resulting from precipitation elasticity (percent change in streamflow per percent change in precipitation) of ~2. We confirm that warming has also driven streamflow declines, as inferred from empirical studies, although operating as a secondary factor. Our finding of a modest −2.5% °C−1 temperature sensitivity, on the basis of our best model-derived estimate, indicates that only about one-third of the attributable climate change signal in Colorado River decline resulted from warming, whereas about two-thirds resulted from precipitation decline.

Grazing-Induced Changes to Biological Soil Crust Cover Mediate Hillslope Erosion in Long-Term Exclosure Experiment

Dryland ecosystems are particularly vulnerable to erosion generated by livestock grazing. Quantifying this risk across a variety of landscape settings is essential for successful adaptive management, particularly in light of a changing climate. In the Upper Colorado River Basin, there are nearly 25 000 km2 of rangelands with underlying soils derived from Mancos Shale, an erodible and saline geologic parent material. Salinity is a major concern within the Colorado River watershed, much of which is attributed to runoff and leaching from Mancos Shale deposits. In a 60-yr paired-watershed experiment in western Colorado, we used silt fences to measure differences in saline hillslope erosion, including both total sediment yield and concentrations of primary saline constituents (Na and Se), in watersheds that were either exposed to grazing or where livestock was excluded. After accounting for the strong effects of soil type, slope, and antecedent precipitation, we found that grazing increased sediment loss by ≈50% across our 8-yr time series (0.1–1.5 tn ha−1), consistent with levels reported at the watershed scale in early published work from studies at the same location. Eroded sediment Se levels were low and unaffected by grazing history, but Na concentrations were significantly reduced on grazed hillslopes, likely due to depletion of surface Na in soils exposed to chronic soil disturbance by livestock. Variable selection and path analysis identified that biological soil crust (BSC) cover, more than any other variable, explained the differences in sediment yields between grazed and ungrazed watersheds, partially through the enhancement of soil aggregate stability. Our results suggest that BSC cover should be granted heightened consideration in rangeland decision support tools (e.g., state-and-transition models) and that measures to reduce surface disturbance from livestock such as altering the timing or intensity of grazing may be effective for reducing downstream impacts.