High-resolution Measurements of Suspended- Sediment Concentration and Grain Size in the Colorado River in Grand Canyon Using a Multi- Frequency Acoustic System

Computation of accurate sediment loads in rivers where the transport of suspended sediment is at least partially regulated by changes in the upstream sediment supply requires high-resolution measurements of suspended-sediment concentration that are collected independently of waterdischarge data. To meet this objective, a multi-frequency acoustic system has been developed and tested on the Colorado River in Grand Canyon, Arizona, USA. This system consists of an array of three single-frequency sideways-looking acoustic-Doppler profilers. Acoustic attenuation is used to measure suspended-silt and clay concentration, and the acoustic backscatter in each frequency is used to measure the concentration of suspended sand in a discrete grain-size range. The median grain size of the suspended sand is then calculated by logarithmic interpolation between the acoustically computed concentrations in each grain-size range. This approach to measuring sediment loads has been shown to be as or more accurate than using conventional sampling methods alone.