Anacostia River Hydrography

Average Daily Discharge (cfs1) Maximum Discharge, June 19722 (cfs) Minimum Discharge, Sept. 1996 (cfs) Surface Area of Tidal River (acres) Average Tidal Time Detention (days) Average Volume of Tidal River (gallons x 109 Average Tidal Rage (feet)
138
31,180
1.8
850
27-36
2.64
2.95
1 cfs (cubic feet per second)= 7.48 gallons per second or 448.8 gallons per minute.
2 Maximum discharge associated with Hurricane Agnes.

The hydrology of the entire Anacostia tributary system may be broadly characterized as being flashy (i.e., quick flow response to rainfall); whereas, the tidal river portion can be described as being sluggish with an average estimated water residence time on the order of 23 to 28 days (Table 1- Anacostia River hydrography). Under periods of extreme low flow, this residence time can be two to three months in duration. Mean daily inflow into the tidal river is approximately 138 cubic feet per second (i.e., 61,934 gallons per minute). As previously inferred, the Northwest and Northeast Branches contribute approximately 93 percent of the river flow. The mean daily baseflow of the Northwest and Northeast Branches is 58and 70 cubic feet per second (cfs), respectively. The sluggish nature of the tidal river causes it to act as a very effective sediment trap. It has been estimated that approximately 85 percent of the incoming sediment load is deposited in the tidal river and remains trapped there (Scatena, 1986).

The 8.4 mile long Anacostia River is fairly shallow, averaging at low tide between three and six feet deep from Bladensburg Marina (Anacostia River Waterfront Park) downstream to the 12th Street Bridge and approximately 10 to 25 feet deep downstream of this bridge to the Potomac River confluence. Examination of Anacostia River depths from the late 1700's (from map prepared by surveyor John Frederick Augustus Prigs, 1790; Lear, 1797) indicates that the River was 30-35 feet deep at its mouth and between 21 to 30 feet deep from the vicinity of what is now the East Capitol Street Bridge (Whitney Young Memorial Bridge) downstream to the Washington Navy Yard.

Wetlands
Approximately 3,208 acres of wetlands remain in the Anacostia, comprising just under three percent of the total watershed area. A majority of this acreage is located in the Coastal Plain portion; that is the eastern two-thirds, of the watershed. Palustrine wetlands make up more than three-quarters of the total wetland acreage in the Anacostia watershed, with the remainder as riverine(20%) and lacustrine (4%).

It is estimated that more than 4,000 acres of nontidal wetlands have been lost from the Anacostia watershed due to both the suburban sprawl of the last five decades and earlier urban development and agricultural activity; this represents greater than 60% of the historical nontidal wetland acreage. More than 90% of the nontidal wetland acreage loss has occurred from the Coastal Plain portion of the watershed and has been concentrated in the lower reaches of the Northwest Branch, the Northeast Branch and Lower Beaverdam Creek subwatersheds.

The loss of tidal wetlands in the watershed has been even more pronounced than that observed for nontidal wetlands. The Army Corps of Engineers estimates that approximately 2,500 acres of tidal emergent wetlands have been destroyed in the Anacostia between Bladensburg and the confluence with the Potomac River. Even with the restoration of Kenilworth Marsh, Kingmon Lake, Heritage Island, River Fringe Wetlands, Bladensburg Marina and Anacostia Site 11(representing approximately 120 acres), less than 180 acres of tidal emergent wetlands currently exist.