Maryland and the District of Columbia:
Surface-Water Resources

U.S. Geological Survey Water Supply Paper 2300, Maryland and District of Columbia

Table of Contents



      Maryland and the District of Columbia both have abundant surface-water resources. In 1980, 72 percent of the population of Maryland and 100 percent of the population of the District of Columbia (table 1) depended on surface water to meet municipal water-supply needs. In 1980, 15,000 Mgal/d (million gallons per day) or 23,200 ft3/s (cubic feet per second) were used to generate hydroelectric power-the primary instream use of surface water. Offstream use primarily is for municipal and industrial supply; in 1980 this water use accounted for 98 percent of offstream surface-water usage in Maryland and 100 percent of offstream usage in the District of Columbia. In Maryland, where ground-water and surface-water resources are used extensively, surface-water withdrawals amounted to 70 percent of the total water withdrawn in 1980. Ground water, which provides the remaining 30 percent of water withdrawals, is used primarily in the Coastal Plain of Mary land where fresh surface-water supplies are less dependable.

      The quantity and quality of surface waters and the mitigation of damages caused by floods are important issues in Maryland and the District of Columbia. State and local governments are being challenged to balance increasing demands by industry and municipalities for additional water supplies to sustain economic growth with the need for recreational areas for an expanding population.


      Maryland is located in the Coastal Plain, the Piedmont, the Blue Ridge, the Valley and Ridge, and the Appalachian Plateaus physiographic provinces (fig. 1). The District of Columbia is located in the Piedmont and the Coastal Plain provinces. The Coastal Plain province, which is underlain by gently dipping unconsolidated strata, rises from sea level to slightly less than 100 feet above sea level east of Chesapeake Bay and to a little more than 200 feet above sea level in southern Maryland. The Piedmont and the Blue Ridge provinces are underlain by crystalline rock and consolidated sedimentary units. The gently rolling hills of the Piedmont have elevations of generally less than 800 feet above sea level. In the Blue Ridge province, elevations rise to more than 1,600 feet above sea level. Sharply folded and faulted consolidated sedimentary strata form the Valley and Ridge province where elevations generally range from about 400 feet in the valleys to about 1,500 feet on ridges. The Appalachian Plateaus are underlain by flat-lying to gently folded sedimentary strata; elevations generally range from 1,500 to 3,000 feet above sea level. The Appalachian Plateaus province contains the highest point in Maryland-3,360 feet above sea level-near the southwestern comer of the State.

      Average annual precipitation in Maryland, based on a 30-year period of record (1951-80), is about 42 inches. In general, precipitation is higher in the eastern and far western parts of the State and lower in the rest of the State (fig. 1). The greatest precipitation (more than 50 inches per year) occurs in the extreme southwestern corner of the Appalachian Plateaus province in western Maryland. The least precipitation occurs just east of the Appalachian Plateaus province where less than 36 inches per year fall because of orographic effects. Average annual precipitation in the District of Columbia is about 43 inches. Precipitation is fairly well distributed throughout the year. The variability of average monthly precipitation at selected rainfall stations in Maryland is shown in figure 1. In general, slightly more precipitation occurs in the spring and summer than in the fall and winter. Throughout the State, average precipitation during all months in the year generally exceeds 2.5 inches.

      The percentage of precipitation that becomes runoff varies considerably over the State. As much as two-thirds of the precipitation that falls on the far western part of Maryland ultimately becomes streamflow. In contrast, only about one-third of the total precipitation in the southeastern part of the State becomes streamflow. In other parts of the State and in the District of Columbia, generally from one-third to one-half becomes runoff. The difference between amounts of precipitation and runoff is made up almost entirely of evapotranspiration losses.

      Runoff varies geographically and seasonally, depending on the geology and the seasonal precipitation patterns (fig. 1). During the winter months of December through February, precipitation falls primarily as snow, and runoff rates are relatively low. During the spring months of March through April, snowmelt, rain, the saturated condition of the soils, and reduced evapotranspiration combine to increase runoff. Runoff during the summer months of June through September is low because of large evapotranspiration losses. During the months of October and November, runoff increases as evapotranspiration declines at the end of the growing season. Examples of the seasonal runoff pattern are shown in figure 1 for the Youghiogheny River near Oakland, the Monocacy River at Jug Bridge near Frederick, and the Choptank River near Greensboro.


      The District of Columbia and almost all of Maryland are within the Mid-Atlantic Region (fig. 2). The Mid-Atlantic Region in Maryland includes the Potomac Subregion (which includes the District of Columbia), the Upper Chesapeake Subregion, and the Susquehanna Subregion. The extreme northwestern corner of Maryland is in the Monongahela Subregion of the Ohio Region. The Potomac, the Susquehanna, and the Monongahela Subregions are dominated by the river basins for which they are named. Large parts of all three basins lie outside of the State; only the Upper Chesapeake Subregion, which includes streams that primarily drain the Coastal Plain province, lies almost entirely in Maryland. These river basins are described below; their location, and long-term average streamflow at representative gaging stations, are shown in figure 2. Streamflow characteristics and other pertinent information are given in table 2.

Potomac Subregion

      The headwaters of the Potomac River drain the mountainous areas of western Maryland, beginning at the State's southwestern corner, and become the North Branch Potomac River-the border between Maryland and West Virginia. The North Branch combines with the South Branch Potomac River, about 20 miles below Cumberland, to form the Potomac River. From there, the river continues for 285 miles as the border between Maryland and West Virginia and, as the border between Maryland and Virginia before it empties into the Chesapeake Bay. The drainage area of the basin at the mouth of the Potomac River is 14,670 mi2 (square miles); 26 percent of the basin is located in Maryland and the District of Columbia. Selected streamflow characteristics are given in table 2 for Conococheague Creek (site 1), Antietam Creek (site 2), and the Monocacy River (site 3)-the major Maryland tributaries to the Potomac River.

      Principal surface-water uses in the Potomac River basin include run-of-the-river hydroelectric facilities; industrial facilities, and public water-supply systems. Bloomington Dam in western Maryland (fig. 2) helps to ensure adequate water supplies for Washington, D.C., during times of drought by making controlled releases to augment natural flows. Construction of the dam was prompted by a severe drought during the summer of 1966, during which flow of the Potomac River at Washington, D.C., dropped to a daily low of 121 ft3/s or 78 Mgal/d after diversions of 480 ft3/s or 310 Mgal/d for municipal use. Projections of population growth for the D.C. area at that time suggested that municipal water needs during another drought of similar magnitude might reduce the flow to levels insufficient to sustain aquatic life.

      Surface-water quality in the basin is related primarily to the type of consumptive use. Extensive agriculture (especially the cultivation of tobacco), coal mining, and increasing industrial activity have resulted in significant erosion, severe siltation, and chemical degradation of the Potomac River. Major sources of the chemical degradation are acid-mine drainage in western Maryland and the discharge of raw sewage from municipal and industrial areas. Many of these problems have been mitigated by upgrading municipal and industrial wastewater-treatment facilities, use of improved landmanagement practices, and reclamation of mined lands.

      Because of the predominantly rural nature of the Potomac River basin, its runoff characteristics are relatively unaffected by human activities (table 2). Bloomington Dam, the largest manmade structure on the Potomac River, regulates drainage from only 266 mi2 of the basin at a point 41 miles upstream from Cumberland. The lake, in addition to small run-of-the-river hydroelectric plants on the main stem, affects low flows in the basin, but has no major effect on flood peaks along the main stem below the confluence of the North and South branches of the Potomac. Major flooding in the Potomac basin occurred in 1889, 1924, 1936, 1937, 1942, and 1972. In the Washington, D.C. area, the flood of March 1936 had a peak flow of 484,000 ft3/s or 313,000 Mgal/d.

      The bar graph for the Monocacy River at Jug Bridge near Frederick (fig. 2, site 3) shows the variability of annual average discharge at the site with time. During the late 1930's and 1940's, average flows were increasing because of periodic flooding. From the late 1940's to the early 1970's, average flows generally declined because of periodic droughts. From about 1972 to 1981, average flows increased because of above-average rainfall. Trends for the Monocacy River are representative of those for the remainder of the Potomac River basin in Maryland and the District of Columbia.

Upper Chespeake Subregion

      The Upper Chesapeake Subregion has a drainage area of approximately 7,400 mi2 in Maryland. This area comprises the major part of the Coastal Plain in Maryland and one-third of the Piedmont province. Principal rivers in the subregion include the Patuxent, the Patapsco, the Gunpowder, the Chester, the Choptank, the Nanticoke, and the Pocomoke. Selected streamflow characteristics for the Pocomoke (site 4), the Choptank (site 5), and the Patuxent (site 6) Rivers are given in table 2.

      The Gunpowder and Patapsco Rivers both have appreciable regulation. Major storage is provided by Liberty Reservoir on the Patapsco River [completed in 1954 with 129,000 acre-ft (acre-feet)) or 42,100 Mgal (million gallons) of storage]; and Prettyboy (completed in 1933) and Loch Raven (completed in 1914) Reservoirs on Gunpowder Falls, with a combined storage capacity of 133,000 acre-ft or 43,300 Mgal. These reservoirs are operated to help meet the municipal water-supply needs of the city of Baltimore and its suburbs. The Patuxent River also has major storage available in the Triadelphia (completed in 1943) and T. Howard Duckett (completed in 1954) Reservoirs, with a combined storage capacity of 36,200 acre-ft or 11,800 Mgal. These reservoirs are used to regulate flood peaks, provide recreational areas, and help meet the water-supply needs of Washington, D.C., and its suburbs.

      The bar graph for the Choptank River near Greensboro (fig. 2, site 5), which shows the long-term variability of annual average discharge for this site, is representative of other unregulated streams in the subregion. Droughts in 1966 and 1977 clearly show the effect of deficient rainfall for extended periods on average discharge. During the drought of 1966, the average monthly flow for August was 5.3 ft3s or 3.4 Mgal/d compared to the long-term August average of 30 ft3/s or 19 Mgal/d. The high annual average discharges of 1952, 1958, and 1972 occurred in response to above average rainfall during those years. A maximum discharge of 6,970 ft3/s or 4,500 Mgal/d occurred during August 1967.

      The Upper Chesapeake Subregion is well known for its agriculture (especially the tobacco industry in southern Maryland) and, more importantly, for the maritime industries supported by the Chesapeake Bay. These industries include shipping, commercial fishing, oy storing and clamming, and harvesting and marketing the world-famous Maryland blue crab. The importance of preserving these industries has led to ongoing programs by Federal, State, and local governments to minimize water pollution caused by rapid urban growth, disposal of industrial and municipal wastes, and agricultural runoff.

Susquehanna Subregion

      Only 282 mi2 of the Susquehanna Subregion is located in Maryland; the remaining 27,187 mi2 of the subregion is in Pennsylvania and New York. The principal river in the subregion is the Susquehanna, which is regulated extensively in Pennsylvania and also in Maryland by Conowingo Dam (fig. 2) (completed in 1928) with a usable storage capacity of 169,000 acre-ft or 55,100 Mgal. Conowingo Dam is operated primarily for hydroelectric power, but it also provides flood control and recreational benefits. The only gaging station in Maryland on the main stem of the Susquehanna River is located at Conowingo Dam. During Hurricane Agnes in June 1972, peak flow at the dam reached 1,130,000 ft3s or 730,000 Mgal/d. Other information on the Conowingo Dam station (site 7) can be found in table 2.

Monongahela Subregion

      The Monongahela Subregion drains 419 mi2 of western Maryland; however, the major part of the subregion is located outside the State. The Youghiogheny is the principal river in the Maryland part of the subregion.

      The Youghiogheny River begins in West Virginia and flows northward into Maryland where it continues northward until it enters Pennsylvania. The largest manmade reservoir in Maryland in the Youghiogheny River basin is Deep Creek Lake (completed in 1925) with a usable storage capacity of 93,000 acre-ft or 30,300 Mgal. The dam that forms the reservoir is on Deep Creek-a tributary to the Youghiogheny River about 7 miles north of Oakland-and provides hydroelectric power to the nearby area. The lake created by the dam is a major recreational attraction for the entire State and is a significant source of revenue during the summer.

      Development of the Monongahela Subregion in Maryland has focused primarily on recreational opportunities and the promotion of tourism; and on industrial exploration, development, and production of natural gas and coal. Agriculture is present throughout the Youghiogheny River basin, but is limited to relatively flat land.

      Runoff characteristics of the Youghiogheny River below the confluence of Deep Creek and Youghiogheny River are affected by the hydroelectric-power generation at Deep Creek Lake. Flow from the lake is totally regulated.

      The bar graph for the Youghiogheny River near Oakland (fig. 2, site 8) is representative of the long-term variability in annual average discharge in the unregulated part of the basin. The general increase in discharge over the period of record may be related to development of recreational areas by the clearing of forested lands and construction of numerous vacation homes and condominiums.


      Two State organizations are responsible for implementing most of the regulatory, planning, and research programs in Maryland. The Maryland Department of Natural Resources through its agencies-the Water Resources Adminstration (WRA) and the Maryland Geological Survey (MGS)-has a major role in surface-water resource planning and management. The WRA provides direction in the development, management, and conservation of water in the State. Its various divisions are responsible for regulation-through permits and management practices related to flood control, erosion and sediment control, watershed development control, dam safety, stormwater control for municipalities, water appropriation for municipal and industrial needs, and mining control. The MGS is responsible for maintaining a statewide water-data network and evaluating the State's water resources. These responsibilities are accomplished in cooperation with the U.S. Geological Survey. The research, data collection, and analyses provided by this cooperative program form an information base upon which surface-water management decisions are made by the WRA.

      The Maryland Department of Health and Mental Hygiene, through its Office of Environmental Programs, is responsible for regulatory and operational programs with regard to the water-quality aspects of surface-water management. As part of these responsibilities, the Office of Environmental Programs issues waster discharge permits and monitors surface-water quality throughout the State.

      In the District of Columbia, one Federal and two local agencies are responsible for managing the surface-water resources. The U.S. Army Corps of Engineers is responsible for developing and maintaining the water-supply source for the District. The District of Columbia Department of Public Works, through its Water and Sewer Utility Administration, is responsible for delivering and metering supplies to users and for repairing the distribution system. The District of Columbia Department of Consumer and Regulatory Affairs regulates permits for withdrawals and disposal of wastewaters, monitors water quality, and handles chemical spills that might adversely affect water supplies.


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District Chief
U.S. Geological Survey
8987 Yellow Brick Road
Baltimore MD, 21237

Prepared by R.W. James, Jr.

Last Modified: 21-May-1999

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