Geology topics

Ronald E. Griffiths

Publications and source records attributed to Ronald E. Griffiths.

At least 19 recordsLinked to original sources

USGS2026

Resurvey of cross sections on the Green River in Browns Park, Colorado and Utah

This study resurveyed ten previously established cross sections and established eight new cross sections on the Green River in Browns Park to document changes in channel width, depth, and area since earlier surveys conducted in 1994. The measured area of the channel cross sections on the Green River in Browns Park generally increased between the initial surveys and 2019. This increase in cross-sectional area was observed in nine of the ten resurveyed cross sections and is indicative of net sediment erosion. The increase in cross sectional area occurred through both channel widening (bank retreat) and increases in depth (bed incision). An analysis of the contribution of bank versus bed changes to the overall area change suggests that the erosion is mostly from the bed of the channel. In addition, weak longitudinal trends in the bed-sand grain-size distribution are consistent with progressive depletion of the sand stored on the bed of the Green River in Browns Park. The findings from our cross-section resurvey support the conclusion that the Green River in Browns Park is experiencing progressive sediment loss and is in a state of sediment deficit.

Colorado, Utah↗

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

Changes in sand storage in the Colorado River in Grand Canyon National Park from July 2017 through June 2020

Changes in the quantity of sand stored within river segments can affect aquatic and riparian habitat, archeological resources, and recreation. Since summer to fall of 2002, gaging stations on the Colorado River in Grand Canyon National Park and on its major tributaries and selected lesser tributaries have measured the mass of sand transported past each station, which allows for changes in the mass of sand stored between gaging stations to be calculated. Sand mass balances on six Colorado River segments are currently measured; the upstream two segments measure sand mass balance in Marble Canyon, the middle three segments measure sand mass balance within the majority of Grand Canyon, and the downstream-most segment—western Grand Canyon and the Lake Mead delta—measures the quantity of sand transported past Diamond Creek and ultimately deposited in Lake Mead. Between July 1, 2017, and June 30, 2020, the amount of sand stored in the Colorado River in Marble Canyon decreased, whereas the sand mass balance in Grand Canyon was indeterminate. Of the 3 years of study presented herein, sand was eroded from Marble Canyon during sediment year 2018 (July 1, 2017–June 30, 2018), a year with less than 40 percent of the 2003–2020 mean Paria River sand input, and sediment year 2020 (July 1, 2019–June 30, 2020), a year with negligible Paria River sand input. During sediment year 2018, when the Little Colorado River supplied negligible sand, sand was also eroded from Grand Canyon. The sand mass balance was indeterminate for Grand Canyon during sediment year 2020. During sediment year 2019 (July 1, 2018–June 30, 2019) sand accumulated in both Marble Canyon and Grand Canyon. This sediment year had sand inputs from both the Paria River and the Little Colorado River of more than 170 percent the 2003–2020 mean, coupled with below post-1964 mean discharge from Glen Canyon Dam.

Arizona↗

USGS2024

Resurvey of cross sections on the Yampa and Little Snake Rivers in Lily and Deerlodge Parks, Colorado

Resurveys of seven geomorphologic cross sections located in the Lily Park and Deerlodge Park, Colorado, reaches of the Yampa and Little Snake Rivers were conducted in October 2017. These cross sections extend from Lily Park, at the confluence of the two rivers, to Deerlodge Park within Dinosaur National Monument. Four cross sections were first surveyed in 1983 and then resurveyed in 1997. The remaining three cross sections were first surveyed in 1997. Analysis of historical aerial photographs (taken from 1961 to 2015) was conducted to contextualize the measured changes in the cross sections, confirm cross-section longitudinal positions along the rivers, and verify the timing of artificial realignment and straightening of the Little Snake River. Erosion occurred between 1983 and 1997 in all four cross sections first surveyed in 1983, largely through channel widening. Continued erosion occurred between 1997 and 2017 in six of the seven cross sections, also largely by channel widening with only minor changes in channel depth. Though erosion occurred over a longer time period, the net erosion observed at these cross sections over three decades is consistent with the net erosion documented by a sediment-transport-based monitoring program on the Yampa River and Little Snake Rivers from 2013 to 2020.

Colorado↗

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

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↗

USGS2023

State of the science and decision support for measuring suspended sediment with acoustic instrumentation

Acoustic instrumentation can be used to provide time-series and discrete estimates of suspended-sediment concentration, load, and sediment particle sizes in fluvial systems, which are essential for creating informed solutions to many sediment-related environmental, engineering, and land management concerns. Historically, scientists have developed relations between suspended sediment characteristics and other parameters, most commonly streamflow, to estimate sediment information when physical sediment samples cannot be collected. Approaches using streamflow can have substantial accuracy limitations because of hysteresis effects, giving rise to the use of more direct surrogate approaches such as acoustic methods. Interagency efforts in recent years have advanced the testing, methods development, operational guidelines, and training on acoustic methods for measuring suspended sediment. Scientists interested in using these methods are faced with many decisions on the type of application and deployment: horizontal profiling, vertical profiling, or point acoustic instruments; single or multifrequency instruments; continuous or discrete sediment measurements; and fixed or mobile instrument deployments. To promote cost-effective, accurate, and high-resolution fluvial sediment data for the Nation, the interagency Sediment Acoustic Leadership Team (SALT) develops technical guidance and training for using acoustic instruments to measure aquatic sediment. Even though acoustic instrumentation has been used successfully to measure suspended-sediment characteristics throughout the world, some deployments have been unsuccessful because of limited technical guidance and selection of an inappropriate method. To guide decisions on method selection, the SALT has compiled the state of the science for the main types of acoustics-based suspended-sediment measurement methods in development, testing, and use, and has created a flowchart to guide method selection.

Conference Paper↗

USGS2023

Acoustic measurements on a shallow, sand-bed river: A case study from the Rio Grande

The Middle Rio Grande (MRG) is a dynamic and complex fluvial system where flow and sediment transported from the Upper Rio Grande and MRG tributaries influence the form of the river. How sediment is transported through the MRG is an important planning question as it addresses a wide range of concerns including flood control and river rehabilitation, thus continuous sediment measurements are needed to develop accurate sediment budgets. Sediment measurement techniques have continued to improve and the advent of sediment surrogates, such as acoustic technology, have proven to be effective options at obtaining more complete spatial and temporal sediment data in larger fluvial systems. Measurements of sediment in shallow, sand bed rivers, like the Rio Grande, are more difficult because of the changing channel morphology and often limited water depth in which to install instrumentation. During the 2019 spring snow-melt runoff season two acoustic techniques were employed on the Rio Grande to evaluate sediment movement. Sediment movement near the bed was calculated by the Integrated Section Surface Difference Over Time version 2 (ISSDOTv2) using swath data collected from a multi-beam sonar. Measurements were made adjacent to U.S. Geological Survey (USGS) gaging stations where near simultaneous measurements were made by the USGS for streamflow, suspended-sediment concentration and gradation, and bed-material gradations. These measurements were conducted at two locations on the Rio Grande, one of the locations was co-located with two side-profiling suspended-sediment acoustic Doppler profilers that had been installed in the fall of 2016. Both a 1 MegaHertz (MHz) and 2 MHz side-profiling suspended-sediment acoustic Doppler instrument were installed on a fixed platform that was co-located with a USGS sediment gage. The ISSDOTv2 method using multi-beam sonar and the side-profiling acoustic Doppler profilers proved successful in collecting sediment information and compared well with the more traditional sediment measurements, while providing insight into the sediment transport on the MRG because of the increase in spatial and temporal resolution. Overall, there are some limitations of these acoustical techniques, but the additional information gleaned is beneficial in understanding sediment transport in a shallow, sand-bed river, such as the Rio Grande.

New Mexico↗

USGS2022

Field investigation of sub-isokinetic sampling by the US D-96-type suspended-sediment sampler and its effect on suspended-sediment measurements

Collection of accurate suspended-sediment data using depth-integrating samplers requires that they operate isokinetically, that is, that they sample at the local stream velocity unaffected by the presence of the suspended-sediment sampler. Sub-isokinetic suspended-sediment sampling causes grain-size dependent positive biases in the suspended-sediment concentration measured by the suspended-sediment sampler. Collapsible bag suspended-sediment samplers like the US D-96 and the lighter US D-96-A1 depth-integrating samplers have shown a tendency to sample sub-isokinetically under low stream velocities (below ~3.5 feet per second), colder water temperatures, and longer sampling durations. Previous work concluded that the time-dependent decrease in the intake efficiency of the US D-96-type sampler could be partially overcome by increasing the venting of water from the sampler cavity by shortening the sampler tray. The standard-length sampler tray partially blocks the rear vent hole; shortening the sampler tray effectively increases the area of the sampler-cavity rear vent hole. This previous work showed that removing the partial blockage of the rear vent hole caused by the sampler tray resulted in both an increase in intake efficiency and a decrease in the positive bias in measured suspended-sand concentration. Herein, a series of tests were conducted on the Colorado River in Arizona using different modifications to a US D-96-A1 sampler to see if physical enlargement of the rear vent hole would produce further improvements in intake efficiency. Results from these tests show that physical enlargement of the rear vent hole, beyond that already effectively achieved by shortening the sampler tray, did not result in any further improvement in intake efficiency. However, these tests also indicated that physically increasing the area of the rear vent hole did not affect the suspended-sediment data collected by the US D-96-A1 sampler. Furthermore, comparisons of suspended-sediment data collected using the US D-96-A1 sampler and the isokinetic US P-61-A1 point-integrating sampler show that the suspended-sediment data collected by the US D-96-type sampler can be accurate in certain circumstances despite the tendency of this sampler to sample sub-isokinetically over the entire depth of a sampling vertical. We surmise that this result could arise from the US D-96-A1 sampler collecting sample isokinetically when the water-sediment mixture enters the nozzle, but that the water-sediment mixture only enters the nozzle intermittently while the sampler transits a sampling vertical.

Arizona↗

USGS2021

Strandlines from large floods on the Colorado River in Grand Canyon National Park, Arizona

Strandlines of peak-stage indicators (such as driftwood logs, woody debris, and trash) provide valuable data for understanding the maximum stage and extent of inundation during floods. A series of seven strandlines have been preserved along the Colorado River in Grand Canyon National Park, Arizona, USA. A survey and analysis of these strandlines was completed from the Colorado River at Lees Ferry, Ariz., gaging station to the Colorado River near Grand Canyon, Ariz., gaging station. Owing to the longitudinally discontinuous nature of the strandlines, several lines of evidence were used to determine the year of the flood associated with each strandline segment. This evidence included strandline relative vertical position, degree of peak-stage indicator weathering, datable trash drift, and map-view location. The seven distinct strandlines identified were deposited during floods with the following peak discharges (in cubic feet per second [ft 3 /s]) at the Colorado River at Lees Ferry, Ariz., gaging station (year of flood in parentheses): 210,000 ft 3 /s (1884), 170,000 ft 3 /s (1921), 125,000 ft 3 /s (1957), 108,000 ft 3 /s (1958), 97,000 ft 3 /s (1983), 52,500 ft 3 /s (1986), and 45,000 ft 3 /s (multiple events between 1996 and 2012). Stage-discharge relations were developed in areas where all, or most of the strandlines were present, and were compared to predicted stage-discharge relations from a one-dimensional flow model. River width exerted a strong control on these relations, with much greater stage change occurring for a given discharge change in narrower bedrock-dominated reaches than in wider reaches with more extensive channel-margin alluvium. This comprehensive dataset allows for the verification of model-predicted flood stage along the Colorado River in Grand Canyon National Park.

Arizona↗

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↗

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↗

USGS2019

Interpreting flux-based sediment budgets in a habitat context: Linking precise temporal-resolution measurements of sediment flux to spatially robust characterization of channel change

Continuous measurements of sediment transport at reach-bracketing gaging stations allow for the construction of continuous mass-balance sediment budgets for the intervening reach. Although these budgets identify periods of sediment surplus (net deposition) or sediment deficit (net erosion), such analyses cannot identify the locations within the reach where channel change occurs. Because channel change and associated changes in habitat are of greater interest to river managers than the precise value of reach-scale loss or accumulation of sediment, it is important to explicitly link reach-scale changes in sediment mass balance to field measurements of channel change. In this study we will evaluate the relationship between the magnitude of the sediment mass imbalance measured by acoustic-Doppler profilers and the resulting channel change on the Yampa River in Dinosaur National Monument.

Conference Paper↗

USGS2018

Long-term evolution of sand transport through a river network: Relative influences of a dam versus natural changes in grain size from sand waves

Temporal and spatial nonuniformity in supplies of water and sand in a river network leads to sand transport that is in local disequilibrium with the upstream sand supply. In such river networks, sand is transported downstream as elongating waves in which coupled changes in grain size and transport occur. Depending on the magnitude of each sand‐supplying event and the interval between such events, changes in bed‐sand grain size associated with sand‐wave passage may more strongly regulate sand transport than do changes in water discharge. When sand transport is controlled more by episodic resupply of sand than by discharge, upstream dam construction may exacerbate or mitigate sand‐transport disequilibria, thus leading to complicated and difficult‐to‐predict patterns of deposition and erosion. We analyzed all historical sediment‐transport data and embarked on a 4‐year program of continuous sediment‐transport measurements to describe disequilibrium sand transport in a river network. Results indicate that sand transport in long river segments can evolve over ≥50‐year timescales following rare large sand‐supplying events. These natural changes in sand transport in distal downstream river segments can be larger than those caused by an upstream dam. Because there is no way to know a priori whether sand transport in a river has changed in response to changes in the upstream sand supply, contemporary continuous measurements of sand transport are required for accurate sand loads and budgeting. Analysis of only historical sediment‐transport measurements, as is common in the literature, may lead to incorrect conclusions with respect to current or future sediment‐transport conditions.

Journal of Geophysical Research: Earth Surface↗

USGS2018

Technical note: False low turbidity readings from optical probes during high suspended-sediment concentrations

Turbidity, a measure of water clarity, is monitored for a variety of purposes including (1) to help determine whether water is safe to drink, (2) to establish background conditions of lakes and rivers and detect pollution caused by construction projects and stormwater discharge, (3) to study sediment transport in rivers and erosion in catchments, (4) to manage siltation of water reservoirs, and (5) to establish connections with aquatic biological properties, such as primary production and predator–prey interactions. Turbidity is typically measured with an optical probe that detects light scattered from particles in the water. Probes have defined upper limits of the range of turbidity that they can measure. The general assumption is that when turbidity exceeds this upper limit, the values of turbidity will be constant, i.e., the probe is pegged ; however, this assumption is not necessarily valid. In rivers with limited variation in the physical properties of the suspended sediment, at lower suspended-sediment concentrations, an increase in suspended-sediment concentration will cause a linear increase in turbidity. When the suspended-sediment concentration in these rivers is high, turbidity levels can exceed the upper measurement limit of an optical probe and record a constant pegged value. However, at extremely high suspended-sediment concentrations, optical turbidity probes do not necessarily stay pegged at a constant value. Data from the Colorado River in Grand Canyon, Arizona, USA, and a laboratory experiment both demonstrate that when turbidity exceeds instrument-pegged conditions, increasing suspended-sediment concentration (and thus increasing turbidity) may cause optical probes to record decreasing false turbidity values that appear to be within the valid measurement range of the probe. Therefore, under high-turbidity conditions, other surrogate measurements of turbidity (e.g., acoustic-attenuation measurements or suspended-sediment samples) are necessary to correct these low false turbidity measurements and accurately measure turbidity.

Hydrology and Earth System Sciences↗

USGS2017

Importance of measuring discharge and sediment transport in lesser tributaries when closing sediment budgets

Sediment budgets are an important tool for understanding how riverine ecosystems respond to perturbations. Changes in the quantity and grain size distribution of sediment within river systems affect the channel morphology and related habitat resources. It is therefore important for resource managers to know if a river reach is in a state of sediment accumulation, deficit or stasis. Many sediment-budget studies have estimated the sediment loads of ungaged tributaries using regional sediment-yield equations or other similar techniques. While these approaches may be valid in regions where rainfall and geology are uniform over large areas, use of sediment-yield equations may lead to poor estimations of loads in regions where rainfall events, contributing geology, and vegetation have large spatial and/or temporal variability. Previous estimates of the combined mean-annual sediment load of all ungaged tributaries to the Colorado River downstream from Glen Canyon Dam vary by over a factor of three; this range in estimated sediment loads has resulted in different researchers reaching opposite conclusions on the sign (accumulation or deficit) of the sediment budget for particular reaches of the Colorado River. To better evaluate the supply of fine sediment (sand, silt, and clay) from these tributaries to the Colorado River, eight gages were established on previously ungaged tributaries in Glen, Marble, and Grand canyons. Results from this sediment-monitoring network show that previous estimates of the annual sediment loads of these tributaries were too high and that the sediment budget for the Colorado River below Glen Canyon Dam is more negative than previously calculated by most researchers. As a result of locally intense rainfall events with footprints smaller than the receiving basin, floods from a single tributary in semi-arid regions can have large (≥ 10 ×) differences in sediment concentrations between equal magnitude flows. Because sediment loads do not necessarily correlate with drainage size, and may vary by two orders of magnitude on an annual basis, using techniques such as sediment-yield equations to estimate the sediment loads of ungaged tributaries may lead to large errors in sediment budgets.

Geomorphology↗

USGS2016

Water clarity of the Colorado River—Implications for food webs and fish communities

The closure of Glen Canyon Dam in 1963 resulted in drastic changes to water clarity, temperature, and flow of the Colorado River in Glen, Marble, and Grand Canyons. The Colorado River is now much clearer, water temperature is less variable throughout the year, and the river is much colder in the summer months. The flow—regulated by the dam—is now less variable annually, but has larger daily fluctuations than during pre-dam times. All of these changes have resulted in a different fish community and different food resources for fish than existed before the dam was built. Recent monitoring of water clarity, by measuring turbidity, has helped scientists and river managers understand modern water-clarity patterns in the dam-regulated Colorado River. These data were then used to estimate pre-dam turbidity in the Colorado River in order to make comparisons of pre-dam and dam-regulated conditions, which are useful for assessing biological changes in the river over time. Prior to dam construction, the large sediment load resulted in low water clarity almost all of the time, a condition which was more favorable for the native fish community.

Arizona, California, Colorado, Nevada Utah, New Me↗