Potomac River Basin

 The below is summarized from the ICPRB Water Resource Plan

The Potomac River, its tributaries, reservoirs and the associated groundwater resources are the source of drinking water for the over 6,000,000 people in the Washington Metropolitan area. The Potomac River and the Occoquan Reservoir are the main supply of water for Fairfax Water which also supplies Prince William Service Authority and American Water. The Potomac River is also the water supply for WSSC and the Washington Aqueduct as well as the new direct supply for Loudoun Water.

Due to its sheer size, the Potomac basin is quite diverse – geographically, ecologically, hydrologically. The 383-mile long Potomac River and has a 14,670 square mile drainage area that includes portions of Virginia (5,723 sq mi), Maryland (3,818 sq mi), West Virginia (3,490 sq mi), and Pennsylvania (1,570 sq mi), and all of the District of Columbia (69 sq mi). There are approximately 16,450 miles of perennial streams in the Potomac basin. They range from fast moving mountain streams with frequent cascades, riffles, and pools to slow moving coastal streams. The river flows generally from west to east- high elevation to low. By either surface or groundwater the Potomac River is connected to 184,944 acres of freshwater and coastal wetlands.

The Potomac River starts at the Fairfax Stone in West Virginia as the North Branch Potomac River. It forms the boundary between Grant, Mineral, and Hampshire counties in West Virginia, and Garrett and Allegany counties in Maryland, for 104 miles before it joins the South Branch Potomac River just downstream from Green Spring, West Virginia. The first 19 miles of the South Branch Potomac River are located in Highland County, Virginia, flowing the remainder of its 139 miles in West Virginia.

After the confluence of the North and South branches, the Potomac River continues for 274 miles as the border between West Virginia and Virginia to the south and Maryland to the north. Ultimately, the Potomac River flows into the Chesapeake Bay at Point Lookout, Maryland. Major tributaries of the Potomac River are,: Cacapon River, Conococheague Creek, Antietam Creek, Shenandoah River, Catoctin Creeks (both of them), Monocacy River, Seneca Creek, Rock Creek, Anacostia River, Occoquan River, and Wicomico River. The river is an estuary for its last 113 miles of its existence. The estuary formed after the last ice age as sea level rose and drowned the river’s channel on the Coastal Plain.

The Potomac River and its tributaries are relatively unregulated compared to other major rivers in the Eastern U.S. That means the river flows mostly free with limited reservoirs. The largest reservoirs in the basin are Jennings Randolph, Occoquan, and Savage River, but there are plans for additional large reservoirs to meet the forecast shortfall in water supply. As the demand for water increases with growth in industrial and residential use and land use changes reduces the recharge of groundwater; the changing climate is forecast to change storm frequency and duration as well as increase the duration of droughts. All these factors will combine to change the demand for water and the flow of the river. The reservoirs that are part of the cooperative management of Potomac drinking water supplies will have to increase so that during times of drought they can be used to ensure adequate flow in the river to serve the population and the environment.

The Potomac basin intersects five major physiographic provinces including, from northwest to southeast, the Appalachian Plateau, Valley and Ridge, Blue Ridge, Piedmont, and the Coastal Plain. In the Coastal Plain Province, groundwater is contained in a confined aquifer system. Recharge of these aquifers primarily occurs by infiltration from overlying aquifers and through outcroppings near the Fall Line. Above the Fall Line, groundwater aquifers consist of fractured bedrock. Fractured bedrock aquifers consist of a thin layer of unconsolidated soil and weathered rock overlying the bedrock. These unconsolidated materials are far more porous than the bedrock and contain the largest volume of groundwater in the fractured rock aquifer. Groundwater is transmitted to wells and streams through the fracture system within the bedrock but there is relatively little storage in the fractures. 

The Potomac basin is heavily forested, with approximately 53 %  forest cover. Agriculture is also a major land use, particularly in the Great Valley and Piedmont regions, and covers 26% of the basin. Fourteen percent of the basin is developed and some regions of the basin, especially Prince William County, are experiencing rapid urbanization which impacts the river flow and water demand.

According to the U.S. Census, the ten Potomac basin locations with the largest increase in impervious cover between 2006 and 2011 are (from highest to lowest): Harrisonburg, Virginia; Manassas Park, Virginia; Winchester, Virginia; Manassas, Virginia; Prince William County, Virginia; Waynesboro, Virginia; Staunton, Virginia; City of Alexandria, Virginia; Prince George's County, Maryland; and Stafford County, Virginia. Changing land use impacts the quality of the Potomac River and the flow. The variability of streamflows in the Potomac basin is primarily a function of temperature, precipitation, evapotranspiration, feed from groundwater, and the relatively free-flowing (unimpounded) nature of the surface waters, but as the river basin changes from open land to developed areas, and the intensity and frequency of precipitation changes with the climate. The river itself and its flow could change.