The Interstate Commission on the Potomac River Basin (ICPRB) conducted a study to describe the biological composition of three under-represented reaches in the mainstem Potomac River Basin and determine the effort required to accurately assess large river sites for freshwater mussel and benthic macroinvertebrate populations. Located at Knoxville (MD), Carderock (MD), and Little Falls (MD), these reaches were selected because they are difficult to sample and represent gaps in spatial coverage of the mainstem in the otherwise comprehensive Maryland Department of Natural Resources (MD-DNR) Core Trend Monitoring Program. Data from the Knoxville reach will improve our understanding of the mixing zones below the confluence of the Shenandoah and Potomac rivers and the relative importance of each river at the Potomac water supply intakes downstream. The Carderock and Little Falls reaches are important in identifying stresses on the river’s biological communities that could relate to upstream consumptive losses and water supply withdrawals during severe droughts. The Little Falls reach is in the only stretch of the Potomac River with a minimum flow-by requirement.

Surveys of freshwater mussel and benthic macroinvertebrate populations were conducted during late-summer low-flow periods of 2012, 2013, and 2014. The three years of the study had moderate flows overall and did not experience extreme drought or floods, so managers and researchers should view the results as a characterization of biological communities unaffected by flow extremes. In addition to recording mainstem Potomac species distributions, biological collections underwent post-collection analyses that provided an informed baseline for the collection effort required to achieve sufficiently accurate data in the future.