This map shows the swampy and estuarial Tiber Creek that ran under most of what is now reclaimed land and constitutes the National Mall. Rock Creek and several smaller tributary streams feed the Anacostia River, on the eastern edge of Andrew Ellicott's layout of streets. These creeks were the sources of fresh water for the city for the first half of the nineteenth century. The government employed army and civilian engineers who were part of the Topographical Corps to survey and purchase rights to water from creeks that ran in the area currently near Howard University. As the concentrations of workers in government office buildings required more water, the original iron pipes led directly from the creeks to the offices around the Capitol, the White House, and along Pennsylvania Avenue. This system was crude and lacked enough pressure to be an adequate guard against fire. The dashed line represents the route of the aqueduct as it splits to serve several areas of the city, including the government offices and the Smithsonian Castle, and then continues in two branches to the Arsenal and Navy Yard along the Anacostia.
Note how the topography, or changes in elevation, affected where Ellicott laid out the city streets from Pierre Charles L'Enfant's more spacious plans. They conform to a natural plateau of flatland at the confluence of the two rivers. Note also how the true "swamp" in this flatland was confined to the area of Swampoodle, a neighborhood that historically had been home to Irish immigrants and is now occupied by the Union Station railyard. Though the Tiber was a slow-moving estuarial creek, one can see that most of the city actually was not built "on a swamp."