Frederick is a city in, and the county seat of, Frederick County, Maryland, United States. Frederick's population was 78,171 people as of the 2020 census, making it the second-largest incorporated city in Maryland behind Baltimore. It is a part of the Washington metropolitan area and the greater Washington–Baltimore combined statistical area.

The Monocacy River is a free-flowing left tributary to the Potomac River, which empties into the Atlantic Ocean via the Chesapeake Bay. The river is 58.5 miles (94.1 km) long, with a drainage area of about 970 square miles (2,500 km2). It is the largest Maryland tributary to the Potomac.