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Persistent overuse of water supplies from the Colorado River during recent decades has substantially depleted large storage reservoirs and triggered mandatory cutbacks in water use. The river holds critical importance to more than 40 million people and more than two million hectares of cropland. Therefore, a full accounting of where the river’s water goes en route to its delta is necessary. Detailed knowledge of how and where the river’s water is used can aid design of strategies and plans for bringing water use into balance with available supplies. Here we apply authoritative primary data sources and modeled crop and riparian/wetland evapotranspiration estimates to compile a water budget based on average consumptive water use during 2000–2019. Overall water consumption includes both direct human uses in the municipal, commercial, industrial, and agricultural sectors, as well as indirect water losses to reservoir evaporation and water consumed through riparian/wetland evapotranspiration. Irrigated agriculture is responsible for 74% of direct human uses and 52% of overall water consumption. Water consumed for agriculture amounts to three times all other direct uses combined. Cattle feed crops including alfalfa and other grass hays account for 46% of all direct water consumption.

Sediment budgets are widely used to measure reach‐scale sediment accumulation and evacuation. Such measurements, however, cannot determine when the disturbance is major and the measured sediment mass imbalance is reflective of a river adjusting to a new equilibrium state, as opposed to situations when the disturbance is minor, and the mass imbalance is reflective of a river adjusting within its existing behavioral regime. Sediment sorting among channels and floodplains can have a large effect on how a river responds to a disturbance. Fine sediment may accumulate in the floodplains while coarser sediment erodes from the channel bed. We demonstrate that if a sediment budget does not account for the different behavior and destination of grain sizes, the budget cannot reveal important channel adjustments. In this study, we evaluated how a sand bed river responded to increases in sediment supply by partitioning a sediment budget among silt/clay and five sand fractions. On average, 12 metric tons/meter (downstream)/year of sand was evacuated from the system, but sorting caused channel margins to behave differently from vegetated islands, revealing how a river can slightly narrow while in deficit. Floodplain shaving and bed coarsening evacuated sediment while channel geometry barely changed, consistent with a river adjusting to a minor disturbance within its behavioral regime. This study is an important reminder that sediment mass imbalance does not always lead to channel change. Mechanisms such as floodplain shaving and bed textural change help rivers absorb minor disturbances and resist channel change. Plain Language Summary: Rivers are experiencing increasing human interference, and global warming will significantly impact the amount of water and sediment rivers must convey, placing an even greater need to predict how rivers respond to disturbance. One important problem is predicting the susceptibility of a river to change. While measuring the amount of sediment that accumulates or evacuates from a reach provides clues to how rivers respond to disturbances, such measurements cannot determine whether reach‐average morphology, such as width, depth, slope, and planform, will significantly change or minor adjustments may occur. In this study, we evaluated how a sand bed river responded to an increase in sediment supply that occurred ∼60 years ago using continuous sediment transport measurements and aerial images to measure channel adjustment. We found that, on average, 12 metric tons/meter (downstream)/year of sand was evacuated from the system, but sediment sorting caused channel margins to behave differently from vegetated islands such that the overall channel characteristics barely changed. This study demonstrates that sediment accumulation or evacuation does not always lead to channel change because other processes related to sand sorting can accommodate disturbances, making sand bed rivers more resistant to change. Key Points: A sediment budget must be segregated by grain sizes found in the channel and floodplain to evaluate morphologic changes in sand bed riversChannel adjustment in sand bed rivers is strongly influenced by sand sorting in channel‐floodplain componentsMass imbalance doesn't necessitate channel change: floodplain shaving and bed texture absorb disturbances with minor channel adjustment [ABSTRACT FROM AUTHOR]

During much of the 21st century, natural runoff in the Colorado River basin has declined, while consumption has remained relatively constant, leading to historically low reservoir storage. Between January 2000 and April 2023, the amount of water stored in Lake Mead and Lake Powell, the two largest reservoirs in the United States, declined by 33.5 million acre feet (41.3 billion cubic meters). As of April 2023, total basin‐wide storage was sufficient to support the 21st century average rate of basin‐wide consumption for only 15 months. Runoff in spring 2023 is predicted to be large, providing a short‐term reprieve. However, it will take four to five additional unusually wet years in succession to refill Lake Powell and Lake Mead if basin‐wide water use remains unchanged. Increasing evapotranspiration and dry soils associated with global climate change makes such a scenario unlikely. To stabilize reservoir storage, basin‐wide use needs to equal modern runoff. To recover reservoir storage, basin‐wide use needs to decline even more. Based on 21st century average runoff, a 13%–20% decline in basin‐wide use would allow for stabilization and some reservoir storage recovery. Future policy debate about reservoir operations will inevitably concern whether most, or all, reservoir storage should be in Lake Mead or in Lake Powell. The choice of one or the other will result in significantly different environmental and recreational outcomes for Glen Canyon and the Grand Canyon. This article is categorized under:Water and Life > Stresses and Pressures on EcosystemsHuman Water > Water GovernanceScience of Water > Water and Environmental Change [ABSTRACT FROM AUTHOR]

Closure of Glen Canyon Dam in 1963 transformed the Colorado River by reducing the magnitude and duration of spring floods, increasing the magnitude of base flows, and trapping fine sediment delivered from the upper watershed. These changes caused the channel downstream in Glen Canyon to incise, armor, and narrow. This study synthesizes over 45 yr of channel-change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations. Although bed lowering in lower Glen Canyon began when the first cofferdam was installed in 1959, most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2.6 m of sediment from the center of the channel. The average grain size of bed material has increased from 0.25 mm in 1956 to over 20 mm in 1999. The magnitude of incision at riffles decreases with distance downstream from the dam, while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream. Analysis of bed-material mobility indicates that the increase in bed-material grain size and reduction in reach-average gradient are consistent with the transformation of an adjustable-bed alluvial river to a channel with a stable bed that is rarely mobilized. Decreased magnitude of peak discharges in the post-dam regime coupled with channel incision and the associated downward shifts of stage-discharge relations have caused sandbar and terrace erosion and the transformation of previously active sandbars and gravel bars to abandoned deposits that are no longer inundated. Erosion has been concentrated in a few pre-dam terraces that eroded rapidly for brief periods and have since stabilized. The abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shill in the stage-discharge relations. In the downstream part of the study area where riffles controlling channel elevation have not incised, channel narrowing has resulted from decreased magnitude of peak discharges and minor post-dam deposition. These physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non-native species. Models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit, the pre-dam surface and subsurface grain size, and the magnitude of post-dam average flows. These results suggest that it is also necessary to consider (1) the possibility of variable responses among different channel elements and (2)the potential importance of exceptional flows resulting from management decisions. Keywords: channel adjustment, erosion, dams. geomorphology, alluvial deposits.

An error model for quantifying the magnitudes and variability of errors generated in the areas of polygons during spatial overlay of vector geographic information system layers is presented. Numerical simulation [...]

The magnitude and frequency of tributary debris flows and the historical range of mainstem river discharges are the main factors that create and modify rapids in the Colorado River system. Monitoring of two recently aggraded debris fans in the Green River canyons of the eastern Uinta Mountains shows that main-stem floods with magnitudes between 40% and 75% of the predam 2 yr flood cause significant reworking of fan deposits. Cutbanks formed at fan margins during both small and large flows, indicating that lateral bank erosion is an important reworking mechanism. Armoring of the debris-fan surface limited the degree of reworking by successive floods, even when subsequent flood magnitudes were similar to those that caused significant reworking. Peak discharges increased the width of the reworked zone, decreased fan constrictions, and lowered the water-surface elevation of the ponded backwater. Contrary to predam geomorphic evidence, monitoring indicated that eroded material from recently aggraded debris fans was deposited in bars adjacent to the downstream parts of both fans. This change in the organization of the fan-eddy complex has the potential to alter the location of recirculating eddies and associated areas of fine-grained sediment deposition and storage. Keywords: eastern Uinta Mountains, debris flows, floods, debris-fan reworking.