Section 303(d) of the federal Clean Water Act (CWA) and the U.S. Environmental Protection Agencys (USEPA) implementing regulations direct each state to identify and list waters, known as water quality limited segments (WQLSs), in which current required controls of a specified substance are inadequate to achieve water quality standards. A water quality standard is the combination of a designated use for a particular body of water and the water quality criteria designed to protect that use. For each WQLS listed on the Integrated Report of Surface Water Quality in Maryland (Integrated Report), the State is to either establish a Total Maximum Daily Load (TMDL) of the specified substance that the waterbody can receive without violating water quality standards, or demonstrate via a Water Quality Analysis (WQA) that water quality standards are being met. The Potomac River Washington County watershed (basin code 02140501) was identified on the Integrated Report under Category 5 as impaired by nutrients and sediments (1996 listings), evidence of biological impacts (2002 listings), methylmercury (impoundment PR Dam no. 4 2002 listing), and polychlorinated biphenyls (PCBs) in fish tissue (2008 listing) (MDE 2008). The methylmercury listing is for the impoundment created by Potomac River Dam no. 4 and all other impairments are listed for non-tidal streams. The 1996 nutrients listing was refined in the 2008 Integrated Report and phosphorus was identified as the specific impairing substance. Similarly, the 1996 suspended sediment listing was refined in the 2008 Integrated Report to a listing for total suspended solids. In 2002, the State began listing biological impairments on the Integrated Report. The current MDE biological assessment methodology assesses and lists only at the Maryland 8-digit watershed scale, which maintains consistency with how other listings on the Integrated Report are made, how TMDLs are developed, and how implementation is targeted. The listing methodology assesses the condition of Maryland 8-digit watersheds with multiple impacted sites by measuring the percentage of stream miles that have an Index of Biotic Integrity (IBI) score less than 3, and calculating whether this is significant from a reference condition watershed (i.e., healthy stream, <10% stream miles degraded). The Maryland Surface Water Use Designation in the Code of Maryland Regulations (COMAR) for the Potomac River Washington County watershed and all tributaries including the Chesapeake and Ohio Canal, Ditch Run, Downey Branch, and Greenspring Run are designated as Use I-P water contact recreation, protection of aquatic life, and public water supply. Camp Spring Run has been designated as Use III-P nontidal cold water and public water supply (COMAR 2009 a, b, c). The Potomac River Washington County Watershed is not attaining its designated use of supporting aquatic life because of biological impairments. As an indicator of designated use attainment, MDE uses Benthic and Fish Indices of Biotic Integrity (BIBI/FIBI) developed by the Maryland Department of Natural Resources Maryland Biological Stream Survey (MDDNR MBSS).