Prettyboy Reservoir is a 1,500-acre (610 ha) reservoir in the Hereford Zone of northern Baltimore County, Maryland. While the reservoir is in Baltimore County, the independent city of Baltimore owns the reservoir and the surrounding land. The reservoir is one of three reservoirs created to supply the municipal water system for Baltimore City, Baltimore County, and northern Anne Arundel County constructed by the Baltimore City Department of Public Works. Prettyboy Reservoir, along with Loch Raven Reservoir further down the Big Gunpowder Falls, provide about 61% of the drinking water for the Baltimore metropolitan area system; for this reason, the Prettyboy is considered a "source water" or drinking water watershed. The reservoir contains about 19 billion US gallons (72,000,000 m3) of water on average.

The Gunpowder River is a 6.8-mile-long (10.9 km) tidal inlet on the western side of the Chesapeake Bay in Maryland, United States. It is formed by the joining of two freshwater rivers, Gunpowder Falls and Little Gunpowder Falls.