Section 303(d) of the federal Clean Water Act (CWA) and the U.S. Environmental Protection Agency's (EPA) 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. For each WQLS, 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 that water quality standards are being met. The Prettyboy Reservoir (basin code 02-13-08-06), located in Baltimore County, MD, was identified on the States list of WQLSs as impaired by heavy metals (1996 listing), nutrients (1996 listing), fecal coliform (2002 listing), methylmercury (2002 listing), and evidence of biological impacts (2002 listing). The heavy metal, nutrient, and methylmercury impairments were listed for the impoundment, and the fecal coliform and biological impairments were listed for the non-tidal streams. This report provides an analysis of recent monitoring data, including hardness data, for heavy metals, which shows that the aquatic life criteria for heavy metals and the designated uses supported by those criteria are being met in the Prettyboy Reservoir impoundment. A water quality analysis (WQA) of heavy metals for the Prettyboy Reservoir impoundment was performed using recent water column and sediment toxicity data.