Geology topics

David J. Dean

Publications and source records attributed to David J. Dean.

At least 19 recordsLinked to original sources

USGS2025

Controls on sediment transport and storage in the Little Snake, Yampa, and Green Rivers in the vicinities of Dinosaur National Monument and Ouray National Wildlife Refuge, Colorado and Utah, with implications for fish habitat in the middle Green River

The transport of sand and finer sediment in the Yampa and Green river network is typically in disequilibrium with the local sediment supply because of the partial decoupling of the sources of water and sediment: most of the water is supplied farther upstream than most of the sediment. This decoupling leads to sand being transported in the main-stem rivers as elongating sand waves following sand resupply during tributary floods. Because of the large amount of sand supplied to the Yampa River by the Little Snake River, Yampa River annual floods generate sand waves that migrate downstream in the Green River causing longitudinal patterns in bed-sand grain size that, in turn, lead to large spatial changes in sand transport. These changes in bed-sand grain size dominate over changes in water discharge in regulating sand transport in the sand-bedded reaches of these rivers. Furthermore, at any given discharge, these changes in bed-sand grain size dominate over all other processes in regulating sand transport in both sand- and gravel-bedded reaches of these rivers. Consequently, erosion or deposition of sand, and the associated changes in fish habitat in the Uinta Basin segment of the Green River are only indirectly related to Green River discharge and Flaming Gorge Dam operations. Owing to the longitudinal patterns of bed-sand grain size associated with the downstream migration of sand waves generated by the Yampa River, a multi-year sequence of large, and likely slightly declining, annual floods on the Yampa River is the probable mechanism that increases backwater fish habitat in the Uinta Basin segment of the Green River. Cross-section resurveys indicate that the Uinta Basin (Jensen to Ouray) segment of the Green River has undergone sand erosion caused by slight channel widening since the 1990s (a channel response in opposition to that observed farther downstream in Canyonlands National Park during this period). These resurveys indicate that sand deposition leads to a decrease in channel complexity whereas sand erosion generally leads to an increase in channel complexity. The backwaters used as native fish nursery habitat consist of deep pools downstream from and adjacent to large bank-attached sandbars; thus, more extensive backwater habitat equates to greater channel complexity. The generation of the sand wave during the first large Yampa River flood in a sequence (that is, the year-1 flood) causes fining of the bed sand near Jensen. The downstream coarsening associated with bed sand that is finer near Jensen than downstream near Ouray causes a downstream decrease in sand transport in the Uinta Basin segment, leading to net sand deposition and decreased channel complexity. Continued downstream migration of this sand wave during the following year’s annual flood (that is, the year-2 flood) then causes downstream fining, leading to erosion of sand and increased channel complexity in this segment. Although the year-1 Yampa River flood supplies the sand and deposits the large sandbars required to form backwaters, and thereby makes possible future backwater habitat, these floods cause a temporary reduction in backwater habitat in the Uinta Basin segment because they tend to cause net sand deposition. It is the subsequent out-year Yampa River floods of likely equal or lesser magnitude that maintain or increase backwater habitat because these are the floods that convey sand through or erode sand from this segment. These typically smaller out-year Yampa River floods rework the sandbars deposited during the year-1 annual flood, thereby leading to the increases in both backwater area and volume that have been measured upon recession of these floods. Although artificial floods released from Flaming Gorge Dam might be used to simulate the habitat maintenance achieved by out-year Yampa River floods, the limited sand supply and stage associated with such dam releases precludes their use as a replacement for the sandbar-depositing role of year-1 Yampa River floods that is a prerequisite for backwater formation in the Uinta Basin segment of the Green River.

Colorado, Utah↗

USGS2025

Channel and floodplain cross-section and bed-elevation analyses of the Green River in Echo, Island, and Rainbow Parks, Dinosaur National Monument, Colorado and Utah

This report by the U.S. Geological Survey focuses on describing geomorphic changes in the alluvial segments of the Green River within Echo, Island, and Rainbow Parks of Dinosaur National Monument, between the 1990s and 2019. Substantial channel change occurred within these meandering alluvial segments following the construction and closure of Flaming Gorge Dam in 1962. Geomorphic analyses in the early 1990s documented this change, but variations in dam operations, climate, and the natural sand supply from the Yampa River have since occurred. It was unclear whether channel change within those meandering alluvial segments had continued since the early 1990s; hence, our study provides an update to previous work. This study used three primary methods to quantify the amount and style of channel change that occurred within those alluvial segments of the Green River: (1) digital aerial-photograph analyses, (2) surveys of alluvial topography in the 1990s and 2019 at fixed cross-section locations, and (3) surveys of channel bathymetry in 1998 and 2019. Our analyses show that channel narrowing has continued, with declines in channel width of 4 percent in Echo Park, 16 percent in Island Park, and 15 percent in Rainbow Park from 1993 through 2019. In 11 of the 15 cross sections examined, vertical accretion of sediment on the floodplain and lateral accretion of sediment on the channel margins led to net sediment deposition and a loss of cross-sectional area. Mean changes in bed elevations showed slight erosion; however, bed elevations were considered stable within the bounds of measurement uncertainty and annual variability within the study area. These results show that channel change has continued to occur in these alluvial segments of the Green River from 1993 through 2019, with the dominant changes including sediment deposition and channel narrowing. Although changes in the operations of Flaming Gorge Dam have occurred, these changes have had little effect on flood peak or duration in the segment of the Green River downstream from its confluence with the Yampa River. Instead, ongoing channel change is likely driven by the amount of sediment supplied from the Yampa River, the duration and magnitude of the combined annual spring snowmelt flood from both the Green River upstream from the Yampa River confluence and the Yampa River, and the capacity of this flood to convey the supplied sediment through these wider alluvial reaches.

Colorado, Utah↗

USGS2024

The effects of vegetative feedbacks on flood shape, sediment transport, and geomorphic change in a dryland river: Moenkopi Wash, AZ

Since the 1950s, Moenkopi Wash, in Arizona, United States, has been transformed from a relatively wide river with little riparian vegetation, to a narrow, heavily vegetated river that is less than half of its former width. We analyzed a ∼95-years-long instantaneous-discharge record, an extensive sediment-transport record, oblique and aerial photographs, historical channel surveys, and historical stage-discharge rating relations to determine the primary mechanisms responsible for this transformation. Frequent, large floods dominated the early part of the discharge record between 1926 and 1940. A dramatic ∼30 % decrease in annual mean discharge, and ∼27 % decrease in the mean of the partial-duration flood series occurred in 1941. This decline did not result in widespread channel change but provided an opportunity for vegetation to establish along the channel margins. Widespread channel narrowing began after a second decline in mean and peak discharge in 1956 at which time the channel banks became heavily vegetated. Between 1952 and 2019, the river channel narrowed by 57–59 %. Approximately 2–4 m of bed aggradation occurred at most study sites. As the channel narrowed and became more vegetated, large floods maintained the narrower channel width but did not cause rewidening. Suspended-sediment transport data illustrate the sediment trapping effects of vegetation that lead to channel narrowing; suspended-sand concentrations decline, thereby indicating sediment deposition, as vegetation becomes progressively inundated during floods. Furthermore, dense channel-margin and floodplain vegetation provide increased roughness, resulting in flood-peak attenuation and alteration of hydrograph shape. Vegetation expansion causes positive feedbacks whereby sediment deposition during floods is exacerbated by the roughness and sediment-trapping effects of vegetation leading to further narrowing. These positive feedbacks have reduced sediment delivery to the Little Colorado River downstream. Channel widening is not likely to occur unless there are very large floods that exceed the erosional threshold of the channel-margin vegetation, or unless large-scale vegetation removal efforts are undertaken.

Arizona↗

USGS2024

U.S. Geological Survey Grand Canyon Monitoring and Research Center: Proceedings of the fiscal year 2023 annual reporting meeting to the Glen Canyon Dam Adaptive Management Program

This proceedings is prepared for the USBR and Glen Canyon Dam Adaptive Management Program (GCDAMP) to account for work conducted and products delivered in FY 2023 by SBSC's Grand Canyon Monitoring and Research Center (GCMRC) and to inform the Technical Work Group of science conducted by GCMRC and its cooperators in support of the GCDAMP. It includes a summary of accomplishments, modifications to work plans, and results related to projects included in GCMRC’s FY 2021-23 Triennial Work Plan. This work was done to support the 11 resource goals identified in the Glen Canyon Dam Long-Term Experimental and Management Plan (LTEMP) Environmental Impact Statement and Record of Decision.

Arizona, Nevada, Utah↗

USGS2023

Colorado River Basin Actionable and Strategic Integrated Science and Technology Project—Science strategy

The U.S. Geological Survey (USGS) conducts a wide variety of science that improves understanding of droughts and their effects on ecosystems and society. This work includes data collection and monitoring of aquatic and terrestrial systems; assessment and analysis of patterns, trends, drivers, and impacts of drought; development and application of predictive models; and delivery of information and decision-making tools to stakeholders. Stakeholders, which include Federal, Tribal, State, and local agencies, nongovernmental organizations, and others, use this information to anticipate, assess, react to, and mitigate drought conditions and impacts. There is no obvious near-term solution to reduce the frequency and severity of droughts or to mitigate drought impacts. Multidecadal drought is a “grand challenge” that benefits from integration of existing technologies, data, knowledge, and models across related and disparate disciplines, facilitated by new science and technology. In response, the USGS initiated a new integrated-science approach in the Colorado River Basin in 2020. The Colorado River Basin was specifically selected because of concerns about future drought and its consequences for the region. This document explains how the Colorado River Basin Actionable and Strategic Integrated Science and Technology (ASIST) project extends and enhances the science supported by USGS Mission Areas and Programs and articulates scientific gaps and stakeholder needs to identify and reduce drought risks. The approach seeks to answer complex Earth science questions developed in partnership with stakeholders about severe long-term drought. An integrated approach is required to tackle these complex questions, which any single science discipline cannot answer on its own. In addition to increased understanding of drought and drought effects in complex systems, the Colorado River Basin ASIST project was designed to improve efficiencies through rapid location of a broad array of data sources, assembly of model-ready multidisciplinary data, and delivery of actionable science to stakeholders at the speeds and scales needed for decision making. The project team identified the following actions needed for USGS to implement and advance an integrated science approach in the Colorado River Basin: (1) engage with stakeholders to document their needs and iteratively co-produce science and science delivery tools to address these needs, (2) integrate monitoring and observation systems developed by USGS and other agencies that track droughts and their effects, (3) collect and provide analysis-ready data to support integrated applications, (4) integrate data and model connections to predict multiple drought impacts, (5) conduct multidisciplinary coordination to improve interpretations, (6) leverage the knowledge base across USGS to enhance decision making, and (7) support the development of new integrated science approaches and technologies that provide analysis and management tools that can be used to adapt to the effects of drought in the Colorado River Basin. Proposals for initial short-term use-case projects were solicited, a subset of which was selected for funding to test implementation of these actions. Additionally, the project organized and convened a series of science and technology collaboration workshops in the USGS focused on challenges that were identified and prioritized by the short-term use-case projects and during the initial stakeholder analysis. These workshops were designed to bring together diverse perspectives to discuss science and technology challenges, stakeholder needs, capabilities, and knowledge gaps, with the goal of determining how the USGS can address challenges, identify future opportunities for continued engagement between participants, and inform the next steps for the Colorado River Basin ASIST project. Continuing to collaboratively engage with a wide range of stakeholders using an integrated approach will provide a suitable foundation of data and tools to formulate actionable intelligence for predicting droughts and informing adaptation to the effects of long-term drought in a holistic, timely, and effective manner.

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

USGS2023

Proceedings of the Fiscal Year 2022 Annual Reporting Meeting to the Glen Canyon Dam Adaptive Management Program

(Hartwell) This report is prepared primarily to account for work conducted and products delivered in FY 2022 by GCMRC and to inform the Technical Work Group of science conducted by GCMRC and its cooperators in support of the Glen Canyon Dam Adaptive Management Program (GCDAMP). It includes a summary of accomplishments, modifications to work plans, results, and recommendations related to projects included in GCMRC’s FY 2021-23 Triennial Work Plan. This work was done to support the 11 resource goals identified in the Glen Canyon Dam Long-Term Experimental and Management Plan (LTEMP) Environmental Impact Statement and Record of Decision (Table 1).

Arizona↗

USGS2022

The effects of requested flows for native fish on sediment dynamics, geomorphology, and riparian vegetation for the Green River in Canyonlands National Park, Utah

Releases of water from Flaming Gorge Dam together with climate-related variations in runoff determine the streamflow regime of the Green River, which affects the physical characteristics of the channel and riparian ecosystem of the Green River corridor in Canyonlands National Park. The dam has decreased peak streamflows and raised base streamflows, resulting in vegetation encroachment and channel narrowing and simplification, which could be detrimental to endangered fish habitats over time. Operations of Flaming Gorge Dam are in part determined by flow recommendations provided by the Upper Colorado River Basin Endangered Fish Recovery Program that are designed to benefit native fish and disadvantage nonnative fish. These recommendations alone may not be sufficient to prevent channel narrowing and simplification. Increases in base flows may contribute to channel narrowing and simplification by increasing the water available to riparian vegetation and reducing the water volume available for increasing peak-flow magnitude or duration This report describes how proposed revisions to these flow recommendations would affect the physical characteristics of the Green River corridor in Canyonlands National Park, with a focus on riparian vegetation and channel width. Hydrologic conditions for the Green River downstream from Flaming Gorge Dam are classified by the U.S. Department of the Interior Bureau of Reclamation as dry, moderately dry, average, moderately wet, or wet. The flow recommendations for peak-flow magnitude and duration in wet years are consistent with geomorphic objectives and historical post-dam flows. In moderately wet years, although the recommended peaks may be sufficient to prevent narrowing over the short term, these peaks are lower than historical post-dam peak flows for moderately wet years and could therefore allow reduction in the occasional large peaks necessary to maintain sediment mobility and channel complexity. For average and drier years, the recommendations allow, but do not require, peak-flow magnitude and durations that are likely to achieve geomorphic objectives.

Utah↗

USGS2022

The use of continuous sediment-transport measurements to improve sand-load estimates in a large sand-bedded river: The Lower Chippewa River, WI

Accurately determining sediment loads is necessary for managing river environments but is difficult because multiple processes can lead to large discharge-independent changes in sediment transport. Thus, estimations of sediment load using discharge–sediment rating curves fit to sparse or historical sediment-transport measurements can be inaccurate, necessitating alternative approaches to reduce uncertainty. Continuous sediment-transport measurements reduce uncertainty because they can be used to detect discharge-independent changes in transport and are therefore unaffected by hysteresis. We used largely continuous approaches to measure sand transport in the lower Chippewa River, a large sand-supplying tributary to the Mississippi River. We used side-looking acoustic-Doppler profilers to continuously measure suspended-sand concentration, and bedform-tracking techniques to episodically measure bedload transport. Bedload transport was then continuously estimated using a discharge-dependent ratio of bedload to suspended-sand transport. This approach allowed determination of sand loads that were not estimated based only on water discharge. Our continuous suspended-sand measurements show that hysteresis between discharge and suspended-sand concentration occurs during most floods. Quasi-continuous bed-elevation measurements using a scour monitor show that lags between discharge and dune geometric adjustment is also common, causing hysteresis between discharge and bedload transport during floods. Furthermore, comparisons of our measurements with historical sediment-transport measurements indicate large discharge-independent declines in both suspended-sand and bedload transport since the 1980s. These findings indicate that sand transport is a non-stationary function of water discharge over timescales ranging from within individual floods to decades. Consequently, although our continuous-measurement approach yields only a ~20–30% improvement over rating-curve estimates of sand load over multi-year periods, our approach yields up to a factor-of-five improvement in sand-load estimates over the shorter, i.e., within a flood, timescales over which the largest discharge-independent changes in sand transport occur.

Wisconsin↗

USGS2021

A river of change—The Rio Grande in the Big Bend region

The Big Bend region is located within the heart of the Chihuahan Desert of North America. Within this region, the Rio Grande, referred to as the Rio Bravo in Mexico, is the international border between the United States and Mexico. The area known as the Big Bend is named after the large northerly bend that the river makes before flowing southeast to the Gulf of Mexico. This region is environmentally protected by both countries. Although large binational conservation efforts exist, the physical and ecological characteristics of the river have been substantially altered. Changes in Rio Grande hydrology (the seasonality, magnitude, duration, and variability in streamflow) have resulted in the widespread physical transformation of the river, resulting in the loss of important habitat for native and endangered fish and increased flood risk. U.S. Geological Survey (USGS) scientists, in cooperation with many other government agencies, universities, and non-governmental organizations (NGOs), are working to better understand these changes to inform management of the Rio Grande.

Texas↗

USGS2021

Instruments, methods, rationale, and derived data used to quantify and compare the trapping efficiencies of four types of pressure-difference bedload samplers

Bedload and ancillary data were collected to calculate and compare the bedload trapping efficiencies of four types of pressure-difference bedload samplers as part of episodic, sediment-recirculating flume experiments at the St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, in January–March 2006. The bedload-sampler experiments, which were conceived, organized, and led by the U.S. Geological Survey’s Office of Surface Water, were part of a broader suite of experiments performed in the rectangular, concrete-lined, sediment-recirculating Main Channel Facility (“main channel flume”). Collectively referred to as “StreamLab06,” the experiments were conducted under the auspices of the National Center for Earth-Surface Dynamics, University of Minnesota. Four pressure-difference-type bedload samplers—a standard Helley-Smith, US BLH-84, Elwha, and Toutle River-2—were deployed by using hand-held rods in the main flume in a series of trials during steady flows as part of the first two of seven phases of the StreamLab06 experiments. The Phase I flows were released over a sand bed. Gravel composed the bed during the Phase II flows. Bedload samples were collected during flows ranging from 2.0 cubic meters per second (near the incipient motion of bed material) to 5.5 cubic meters per second. A total of 2,030 bedload samples were collected—1,000 as part of 19 sand-bed trials, and 1,030 as part of 27 gravel-bed trials. Bedload was captured in five contiguous weigh drums inside a slot spanning the full width of the main flume channel 8.5 meters downstream from the cross-section in which the bedload samplers were deployed. The contents of each drum were automatically weighed and recorded as a time series about every 1.1 seconds. Each drum automatically, independently, and episodically dumped its contents into the bottom of the slot upon the accumulation of a pre-determined mass of entrapped sediment, after which the drum continued to capture and weigh bedload. An auger at the bottom of the slot evacuated the accumulating sediment to a side-channel pump that piped the captured sediments upstream and discharged them back to the flume. Bedload-transport rates were calculated from measurements of the masses of material trapped by the bedload samplers and from the data produced by the automated bedload capture-and-weigh system of the main channel flume. These data were used to compute at-a-point and mean bedload-transport rates for subsequent use in developing bedload-trapping efficiency (calibration) coefficients for each bedload sampler and for comparing the relative trapping efficiencies of the manually deployed bedload samplers. The data were collected to enable the use of several computational methods for deriving bedload-trapping coefficients. Continuous ancillary data including stage, water discharge, and water temperature were automatically collected and stored. Flow depths were manually measured and recorded concurrent with each at-a-point bedload-sampler deployment. Other information obtained during parts of the experiments included longitudinal water-surface slope, bedload particle-size distributions, and suspended-sediment concentrations and percent sand analyzed from samples collected by depth integration with a US DH-48 isokinetic suspended-sediment sampler. This report describes the types and availability of the bedload and ancillary data derived through the StreamLab06 experiments. The data are available from the St. Anthony Falls Laboratory and the U.S. Geological Survey through a data release. Also included are selected descriptive and historical information as well as the background, experimental design, experimental caveats, and other factors relevant to the production of the bedload-transport and ancillary data produced through Phases I and II of the StreamLab06 experiments.

Open-File Report↗

USGS2021

Self-limitation of sand storage in a bedrock-canyon river arising from the interaction of flow and grain size

Bedrock-canyon rivers tend to be supply limited because they are efficient transporters of sediment and not because the upstream supply of sediment is small. A byproduct of this supply limitation is that the finer alluvium stored in these rivers has shorter residence times and smaller volumes than in alluvial rivers. To improve our understanding of disequilibrium sediment transport and its effect on sand storage in bedrock-canyon rivers, we undertook a 20-year study, synthesized herein, of the Colorado River in Grand Canyon. Despite the large loads for which it was renowned, this river exhibited evidence of natural sand-supply limitation and became the perfect natural laboratory for studying sand transport in a bedrock canyon after upstream dam construction exacerbated this supply limitation. During our study, we made and analyzed an unprecedented ∼2.5 million measurements of the suspended and bed sediment. Results indicate that sand storage in this bedrock-canyon river is self-limiting owing to the physical controls of flow and grain size causing negative feedbacks that likely also operate in other bedrock-canyon rivers. Following episodic tributary floods that supply finer sand, sand migrates quickly downstream in the form of a wave in which large systematic changes in bed-sand grain size occur. These grain-size changes cause discharge-independent systematic changes in suspended-sand concentration in excess of a factor of 20. Although the tributary supply of sand increases the amount of sand storage, it also greatly increases the downstream sand transport by causing bed-sand fining, thus limiting the residence time and volume of sand storage.

Arizona↗

USGS2021

Assessing the hydrologic and physical conditions of a drainage basin

An assessment of a drainage basin and its stream corridor will provide the data and information needed to understand current biophysical conditions and trends. Developing an understanding of the drivers of change is the next essential step for restoration success (Osterkamp and Toy, 1997; Corenbilt et al., 2007; Briggs and Osterkamp, 2003), Shields et al. 2003; Osterkamp et al., 2011). Establishing such a robust scientific foundation will allow stream practitioners to develop realistic restoration objectives and the tactics that will be effective to achieve them. Accomplishing this requires collecting data at watershed and reach scales and by drawing on scientific data from areas where conditions may be similar to, or applicable to, your site. Although decisions should be backed by conclusive data, to make progress we need to rely on the best available information, even if scientific uncertainty remains. Additional information will become available, so it is necessary to plan a project in a manner that incorporates available knowledge and permits goals, objectives, and tactics to be adjusted accordingly.

Book chapter↗

USGS2021

Monitoring the results of stream corridor restoration

Often overlooked and underfunded, ecological monitoring is an essential component of stream-restoration work. It helps practitioners to identify successful restoration practices, detect ineffective ones, and adjust their adaptive-management activities to improve efficacy (Bernhardt and Palmer 2011). Monitoring, along with research and modeling, are the three legs of the scientific stool that support ecosystem restoration and management. Monitoring tells us what is happening, research tells us why and how it is happening, and modeling provides insights about what can happen under different management alternatives.

Book chapter↗

USGS2020

Does channel narrowing by floodplain growth necessarily indicate sediment surplus? Lessons from sediment‐transport analyses in the Green and Colorado rivers, Canyonlands, Utah

Analyses of suspended sediment transport provide valuable insight into the role that sediment supply plays in causing geomorphic change. The sediment supply within a river system evolves depending on the discharge, flood frequency and duration, changes in sediment input, and ecohydraulic conditions that modify sediment transport processes. Changes in supply can be evaluated through analyses of coupled changes in suspended sediment concentration and grain size. The concentration of sand in transport in the Green and Colorado Rivers is most strongly controlled by discharge and the bed sand grain size distribution. Since the 1950s, sand loads have decreased in response to declines in peak discharge in the Green River and coarsening of the bed sand in the Colorado River. However, changes in the bed sand grain size distribution are associated with large changes in suspended sand concentration in both rivers; concentration varies by a factor of ~3 in the Green River and a factor of ~8 in the Colorado River, depending on the bed sand grain size distribution. Analyses of hysteresis in suspended sediment measurements show that sediment depletion during annual floods is most strongly controlled by flood duration, with peak discharge being nearly equally important in the Green River. Despite channel narrowing in both rivers, periods of bed sand coarsening and sediment depletion during annual floods indicate that these rivers are not necessarily in sediment surplus. Channel narrowing appears to be strongly controlled by short‐term declines in flood magnitude and the ecohydraulic effects of vegetation and may not be indicative of the long‐term sediment budget.

Utah↗

USGS2020

Channel narrowing by inset floodplain formation of the lower Green River in the Canyonlands region, Utah

The lower Green River episodically narrowed between the mid-1930s and present day through deposition of new floodplains within a wider channel that had been established and/or maintained during the early twentieth century pluvial period. Comparison of air photos spanning a 74-yr period (1940−2014) and covering a 61 km study area shows that the channel narrowed by 12% from 138 ± 3.4 m to 122 ± 2.1 m. Stratigraphic and sedimentologic analysis and tree ring dating of a floodplain trench corroborates the air photo analysis and suggests that the initial phase of floodplain formation began by the mid-1930s, approximately the same time that the flow regime decreased in total annual and peak annual flow. Tamarisk, a nonnative shrub, began to establish in the 1930s as well. Narrowing from the 1940s to the mid-1980s was insignificant, because floodplain formation was approximately matched by bank erosion. Air photo analysis demonstrates that the most significant episode of narrowing was underway by the late 1980s, and analysis of the trench shows that floodplain formation had begun in the mid-1980s during a multi-year period of low peak annual flow. Air photo analysis shows that mean channel width decreased by ∼7% between 1993 and 2009. A new phase of narrowing may have begun in 2003, based on evidence in the trench. Comparison of field surveys made in 1998 and 2015 in an 8.5 km reach near Fort Bottom suggests that narrowing continues and demonstrates that new floodplain formation has been a very small proportion of the total annual fine sediment flux of the Green River. Vertical accretion of new floodplains near Fort Bottom averaged 2.4 m between 1998 and 2015 but only accounted for ∼1.5% of the estimated fine sediment flux during that period. Flood control by Flaming Gorge Dam after 1962 significantly influenced flow regime, reducing the magnitude of the annual snowmelt flood and increasing the magnitude of base flows. Though narrowing was initiated by changes in flow regime, native and nonnative riparian vegetation promoted floodplain formation and channel narrowing especially through establishment on channel bars and incipient floodplains during years of small annual floods.

Utah↗

USGS2020

The roles of flood magnitude and duration in controlling channel width and complexity on the Green River in Canyonlands, Utah, USA

Predictions of river channel adjustment to changes in streamflow regime based on relations between mean channel characteristics and mean flood magnitude can be useful to evaluate average channel response. However, because these relations assume equilibrium sediment transport, their applicability to cases where streamflow and sediment transport are decoupled may be limited. These general relations also lack the specificity that is required to connect specific characteristics of the streamflow and sediment regime with the dynamics of channel morphological change that create channel complexity, which is often of ecological interest. We integrate historical records of channel change, observations of scour and fill during a snowmelt flood, measurements of sediment transport, and predictions from a two-dimensional streamflow model to describe how annual peak flow magnitude and peak-flow duration interact with the upstream sediment supply to control channel form for a 15-km study reach on the regulated Green River in Canyonlands National Park, Utah. Two major decadal-scale episodes of channel narrowing have occurred within the study area. For each of these episodes, the reduction in average channel width was consistent with the change predicted by hydraulic geometry relations as a function of average flood magnitude. However, channel narrowing occurred during periods of exceptionally low annual floods. The most recent episode of channel narrowing occurred between 1988 and 2009, during low-flow cycles when the 5-yr mean peak flow was less than 60% of the long-term (1959–2016) mean peak flow. These findings, together with findings from previous studies, demonstrate that decreases in peak-flow magnitude caused by streamflow regulation, climate change, or a combination of those factors have driven episodes of channel narrowing on the Green River. Observations of streamflow, sediment-transport, and morphologic change coupled with predictions from a two-dimensional streamflow model indicate that peak flow magnitudes of at least 75% of the long-term mean peak flow are required to transport bed-material sand in suspension in all regions of the multi-thread channel and that the ~2-month duration of the snowmelt flood played an important role in creating conditions necessary to maintain channel conveyance. These results indicate that detailed characterizations of channel response such as these are needed to predict how river channels will respond to changes in streamflow regime that affect annual peak flow magnitude and duration.

Utah↗

USGS2019

Geomorphic change and biogeomorphic feedbacks in a dryland river: The Little Colorado River, Arizona, USA

The Little Colorado River in Arizona, U.S.A. has undergone substantial geomorphic change since the early 1900s. We analyzed hydrologic and geomorphic data at different spatial and temporal scales to determine the type, magnitude, and rate of geomorphic change that has occurred since the early 20th century. Since the 1920s, there have been 4 alternating periods of high and low total-annual flow. Peak-flow magnitude, however, has progressively declined. In some reaches, the channel has narrowed between 72 and 88% since the 1930s. Increases in sinuosity in wide alluvial valleys have resulted in reductions in channel slope by ~21 to 32%; channel bed aggradation up to 1.4 m has also occurred in some reaches. Newly developed floodplains have been colonized by dense stands of vegetation that appear to have stabilized these surfaces. Large, long duration floods may cause some channel widening, and meander migration, however, these floods are infrequent, and narrowing resumes shortly thereafter. Channel narrowing, increases in sinuosity, decreases in slope, and increases in vegetative roughness appear to have caused biogeomorphic feedbacks, thereby exacerbating sediment deposition, and disrupting flood conveyance. In recent decades, there has been an increase in the travel time of floods up to ~100% compared to floods of the 1940s and 1950s, and this has likely led to increased flood attenuation, contributing to decreases in peak-flow magnitude. The progressive increase in water development in parts of the basin has also likely played some role in the progressive declines in peak flow over the duration of the study.

Arizona↗

USGS2019

Optimal timing of high-flow experiments for sandbar deposition

Sediment-transport theory and field measurements indicate that the greatest or most efficient deposition of sand in eddies occurs during controlled floods (a.k.a. High-Flow Experiments or HFEs) when the greatest amount of the finest sand is available on the bed of the Colorado River (Topping and others, 2010). Conducting HFEs when the sand on the bed of the Colorado River is depleted and coarse can result in relatively widespread erosion of sandbars during HFEs (Hazel and others, 1999; Schmidt, 1999, Rubin and others, 2002). Here we show that sandbar building during HFEs is maximized during periods following tributary floods that resupply the river with large amounts of very fine sand. Conversely, sandbars erode during HFEs when the antecedent sand supply is depleted and coarse. HFEs should be conducted during the fall-winter months of October through January to take advantage of having the greatest amount of very fine sand available on the bed of the Colorado River in Marble Canyon. Conducting HFEs in the spring would necessitate lowering dam operations over the winter months in order to retain the very fine sand supplied during the previous summer.

Arizona↗