Lake Needwood is a 75-acre (300,000 m2) reservoir in Derwood, Maryland, USA. It is just east of Rockville, in the eastern part of Montgomery County, and is situated on Rock Creek. The lake was created to provide flood control. It also protects the water quality of the creek by functioning as a retention basin to trap sediment from stormwater runoff. Ironically, there have been several evacuations of downstream residents during periods of heavy rain, due to concerns about the structural integrity of the earthen dam constructed in 1965.