Suspended-Sediment Transport Dynamics of the Rio Grande/Rio Bravo

Science Center Objects

The Rio Grande/Rio Bravo (hereafter referred to as the Rio Grande) in the Big Bend region of Texas, USA, and Chihuahua, and Coahuila, MX has substantially narrowed since the early 1900s. This narrowing has been caused by the construction and operation of dams and irrigation diversions in upstream reaches of the Rio Grande in the U.S. and the Rio Conchos in Mexico that has reduced mean and peak flows without similar reductions in the amount of sediment contributed to the river channel. Channel narrowing has been exacerbated by the invasion on non-native riparian vegetation that stabilizes banks and traps additional sediment. Current management efforts are aimed at reducing the degree of channel narrowing such that important in-channel habitats are maintained. To address this goal, we use acoustic technology to continuously monitor suspended-sediment-transport to determine whether operations of upstream dams can help alleviate conditions of sediment surplus, and thereby limit channel narrowing. The goals of these efforts are to inform regional environmental managers with potential rehabilitation options.

Background & Importance

This project uses state-of-the art acoustic technology to continuously monitor suspended-sediment transport on the Rio Grande in Big Bend National Park. Suspended-sediment transport is also conducted on two large tributaries to the Rio Grande. Using these data, the amount of fine sediment contributed to the Rio Grande can be determined, and the effects of upstream dam operations on evacuating sediment from the river channel can be evaluated. This information provides the opportunity for river managers to understand how different dam operation scenarios affects sediment transport and geomorphic form, both of which control the quality and quantity of available aquatic habitat.  

Repeat photography from about 1945 (top panel) and 2008 (bottom panel) showing channel narrowing over time

Repeat photography from about 1945 (top panel) and 2008 (bottom panel) showing channel narrowing over time in a portion of the Rio Grande in Big Bend National Park. (Credit: Unknown (top panel); David Dean (bottom panel), unknown (top panel); USGS (bottom panel). Public domain.)

General Methods

Suspended-sediment transport is monitored using side-looking acoustic Doppler profilers in two frequencies. Acoustic attenuation (sound loss) is used to calculate suspended silt-and-clay concentration within the river cross section, acoustic backscatter (scattering of sound) is used to calculate the apparent suspended-sand concentration in each frequency, corrected for the backscatter produced by silt and clay, and, the apparent suspended-sand concentration calculated at each frequency is used to calculate a two-frequency measure of the suspended-sand concentration and median grain size in the river cross section.

Acoustic data are calibrated with physical depth-integrated suspended-sediment samples and samples collected using automatic pump-samplers. Suspended-sediment concentration data are integrated over the river hydrograph to calculate suspended-sediment loads. Suspended-sediment transport data at multiple stations, combined with inputs from tributaries, are then used to create a suspended-sediment budget to determine the amount of sediment that has accumulated or been evacuated from the river reach of interest. Suspended-sediment concentrations, grain sizes, loads, and a combined suspended-sediment budget are all available at the following website: http://www.gcmrc.gov/discharge_qw_sediment/.

Important Results

Results from the this study shows that increases in the in-channel sediment supply primarily occur when short-duration, tributary-sourced, flash floods enter the mainstem Rio Grande. These floods quickly attenuate within the Rio Grande, and large proportions of the tributary-derived load are deposited within the mainstem river. This causes fining of the channel bed and increases in the areal extent of fine sediment on the channel bed. In contrast, fine-sediment accumulation is small or fine sediment is eroded when moderate-magnitude, long-duration, upstream dam releases occur. The effectiveness of dam releases in exporting fine sediment is dependent not only upon the discharge and duration of the dam release, but also upon the antecedent sediment-supply within the channel. The greatest erosional efficiency on the Rio Grande is achieved when moderate-magnitude dam releases occur following periods when there have been many tributary-sourced flash floods, and large amounts of fine sediment have been input into the channel of the Rio Grande.