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The Potomac River /pÉËtoÊmÉk/ is located along the mid-Atlantic coast of the United States and flows into the Chesapeake Bay. The river (main stem and North Branch) is approximately 405 miles (652 km) long, with a drainage area of about 14,700 square miles (38,000 kmÂ²).n terms of area, this makes the Potomac River the fourth largest river along the Atlantic coast of the United States and the 21st largest in the United States. Over 5 million people live within the Potomac watershed.
The Potomac River basin study will focus on the freshwater part of the river upstream from Indian Head, Md. The average flow of the Potomac near Washington, D.C., is 7,440 million gallons per day for the period 1930-89.he Potomac River basin is underlain by many different types of rocks. The rocks in the mountainous Appalachian Plateaus and Valley and Ridge provinces are sedimentary (sandstones, shales, and limestones). The mountainous Blue Ridge and rolling Piedmont provinces are underlain mostly by crystalline rocks.