The term 'River of Washington' is referring to the Potomac River, which is a 7-letter river located in the eastern United States. It runs through Washington D.C., Maryland, Virginia, and West Virginia, and is one of the most important rivers in the region. The Potomac River is over 400 miles long and is a major tributary of the Chesapeake Bay. It is also an important source of recreation, providing opportunities for fishing, boating, and other outdoor activities.