BACKSCATTER AS A SURROGATE FOR SUSPENDED SEDIMENT, NUTRIENT, AND BACTERIA CONCENTRATIONS IN AN URBAN STREAM WITHIN ROCK CREEK NATIONAL PARK, WASHINGTON D.C.

Anthropogenic and naturogenic processes continuously morph landscape and stream geomorphology within Rock Creek National Park, in Washington D.C. To accurately model suspended sediment, nutrient, and bacteria concentrations in Rock Creek, a robust surrogate is required. Robust surrogates are collected at frequencies that capture change, are absent of hysteresis effects, are capable of measuring large sampling volumes, and are resistant to physical and biological fouling. Backscatter (a measure of the reflectance of waves back to a source) from acoustic Doppler velocity meters (ADVM), operating on the principles of sound, is used as a robust surrogate for estimating continuous suspended sediment, nutrient, and bacteria concentrations over periods when discrete samples are not collected. However, due to rapidly changing conditions within urban streams, innovative approaches are required to install and collect backscatter data from deployed ADVM. In October 2014, the United States Geological Survey (USGS) began collecting velocity and backscatter data from two ADVM deployed in Rock Creek at the Joyce Road station to use as a surrogate for concentrations of suspended sediment, nutrients, and bacteria. ADVM and communication equipment were configured accordingly to take full advantage of ADVM multicell capabilities and provide two-way telemetry. Models derived using the USGS Surrogate Analysis Index Developer (SAID) tool returned backscatter as a significant (p-value < 0.1) predictor variable for estimating in-stream concentrations of suspended sediment, total phosphorus, and Escherichia coli. Computed time series will be displayed to the public on the web at http://nrtwq.usgs.gov/md/. The use of multiple acoustic frequencies, 3000 and 1500 kHz, to measure suspended sediment grain size is currently being explored.