NEAR-CONTINUOUS SUSPENDED SEDIMENT MONITORING OF THE RIO GRANDE USING MULTI-FREQUENCY ACOUSTIC INSTRUMENTATION in BIG BEND NATIONAL PARK, USA

The Rio Grande in the Big Bend region of the USA and Mexico is in disequilibrium. The river in this reach rapidly narrows during low-flow years and widens during rare, large floods. One management strategy to improve in-channel habitat for the native ecosystem is to limit the rate and magnitude of channel narrowing during low-flow years by maximizing fine sediment transport downstream using water releases from re-operated upstream dams. A suspended-sediment monitoring program consisting of two suspended-sediment gages was established in 2010 at two sites in Big Bend National Park to inform river management planning. Suspended-sediment gages consist of two single-frequency sideways-looking acoustic-Doppler profilers that collect data at 15-minute intervals. Acoustic attenuation is used to calculate silt-and-clay concentration, and acoustic backscatter is used to calculate sand concentration in two size classes. Acoustic attenuation and backscatter are calibrated to standard depth-integrated samples automatic pump samples. Analyses during a steady, 8-day dam release indicate that there were different sediment transport dynamics at the two gages. At the upstream gage near Castolon, sand transport appeared to be regulated largely by changes in the upstream supply; as discharge increased, suspended-sand concentration increased while the grain-size distribution fined, thereby indicating that a new source of finer sand was being accessed by the river during this release. At the downstream gage near Boquillas, sand transport was regulated mostly by changes in discharge, with the median grain size of suspended sand positively correlated with both discharge and sand concentration. Silt and clay transport was comparable at the two sites during the release. These data suggest that dam releases have the potential to export sediment from the study reach. In contrast, Rio Grande floods originating in tributaries tend to attenuate and be net depositional. When tributary flash floods occur during periods of main-stem low-flow, more than 95% of the contributed load of silt and clay is retained in the study reach. These conclusions provide insight that may allow better management of sediment resources in the Rio Grande in BBNP, and would not be possible without the use of high-resolution suspended-sediment monitoring.