Dalecarlia Reservoir
Dalecarlia Reservoir Show map of the District of Columbia Dalecarlia Reservoir Show map of the United States
Dalecarlia Reservoir
Location	Bethesda, Maryland /Washington, D.C., U.S.
Coordinates	38°56′33.0216″N77°6′36.1548″W / 38.942506000°N 77.110043000°W /38.942506000; -77.110043000
Type	Reservoir
Primary inflows	Washington Aqueduct
Basin countries	United States
Surface area	50 acres (20 ha)
Surface elevation	148 ft (45 m)[1]

Dalecarlia Reservoir is the primary storage basin fordrinking water inWashington, D.C., andArlington County, Virginia. Thereservoir is fed by an undergroundaqueduct in turn fed by lowdams which divert portions of thePotomac River nearGreat Falls andLittle Falls.^[2] The reservoir is located betweenSpring Valley and thePalisades, two neighborhoods inNorthwest Washington, D.C., andBrookmont, a neighborhood inMontgomery County, Maryland.

The 50-acre (200,000 m²) reservoir was completed in 1858 by theUnited States Army Corps of Engineers (USACE) as part of theWashington Aqueduct project.^[3] It began providing water on January 3, 1859.^[4] Initially the reservoir provided water to the city from the adjacentLittle Falls Branch until the aqueduct construction was completed. Regular water service from the Potomac River source through the aqueduct commenced in 1864. The reservoir was modified in 1895 and 1935 to improve water quality and increase water supply.^{[5]: 75, 99}

In the 1920s awater purification plant was built adjacent to the reservoir. Therapid sand filter plant began operation in 1927.^{[5]: 101–105 [6]}

In 1942, the headquarters of theArmy Map Service was established on the grounds adjacent to the reservoir; several buildings constructed in the 1940s still exist. In 1946, its headquarters moved to the nearbySumner Site, which is today theIntelligence Community Campus-Bethesda.^[7]

Until the early 21st century, the semi-solidresiduals (sludge) produced by the treatment plant were periodically discharged to the Potomac River. TheU.S. Environmental Protection Agency (EPA) required USACE to halt most of these discharges, and a residuals handling facility was built on site, which went into operation in 2012.^[8][9]

The reservoir and water treatment plant are operated by the Washington Aqueduct, Baltimore District, U.S. Army Corps of Engineers.^[10]

According to theKöppen Climate Classification system, the area has ahumid subtropical climate, abbreviated "Cfa" on climate maps. The hottest temperature recorded at Dalecarlia Reservoir was 105 °F (40.6 °C) on August 17, 1997, while the coldest temperature recorded was −11 °F (−23.9 °C) on January 21, 1985.^[11]

• District of Columbia Water and Sewer Authority
• Georgetown Reservoir
• McMillan Reservoir

• Bethesda, Maryland
• Brookmont, Maryland
• Lakes of Montgomery County, Maryland
• Potomac River watershed
• Reservoirs in Washington, D.C.
• Reservoirs in Maryland
• The Palisades (Washington, D.C.)
• United States Army Corps of Engineers

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