Washington, D.C., isn't built on a swamp. However, much of the National Mall and the southern part of downtown to about Pennsylvania Avenue NW is in a floodplain. After an exceptionally severe flood in 1881, the federal government decided to dredge the Potomac River down to bedrock to provide a deeper channel for floodwaters. They piled the recovered soil against the shore. There was so much of it, that vast amounts of new land was created ("reclaimed", in the parlance): Everything of the National Mall south of Constitution Avenue and west of the Washington Monument; all of West Potomac Park; all of East Potomac Park; and the Tidal Basin.
The new land was six feet higher than the old. Berms were created along Constitution Avenue and around the Washington Monument to help hold back any floodwaters that did make up over the shoreline.
In 2006, a major study showed that the flood danger due to climate change would overwhelm the 1900 defenses. The Army Corps of Engineers has spent the last three years building giant underground cisterns to help catch these stormwaters. New, six-foot-high flood retaining walls are now being built, and soon dirt will be back-filled against them.
Because there are some major north-south running streets, the new flood walls are designed to have a big metal plate slid into them -- blocking off these streets, and keeping the floodwaters back.
17th Street NW was blocked off today for construction of these new walls. Check it out!
Frankly, the cisterns are too small to hold back the Potomac. They're really there to drain water away from Federal Triangle -- where the basements are full of water, due to the existence of Tiber Creek under Constitution Avenue.
The flooding of the Federal Triangle is NOT due to hurricanes or massive thunderstorms, but rather due to the existence of Tiber Creek -- half of which is currently buried in a sewer tunnel beneath Constitution Avenue and half of which is still flowing through the surrounding earth north of Constitution Avenue. In fact, the basement levels of most of the buildings on the south side of Federal Triangle leak: Every day, eight buildings -- including the Commerce Department and the FBI -- collectively pump 1.7 million gallons of groundwater from their basements because of submerged Tiber Creek. The east wing of the Smithsonian's National Museum of Natural History is pulling away from the main building because Tiber Creek is beneath it. Stormwater floods these basements even more -- not because the city sewer system cannot handle the runoff. It can: There is an overflow system that dumps stormwater directly into the Potomac River (a massive problem, BTW). The problem is the existence of Tiber Creek, which leaves the ground below and north of Constitution Avenue so saturated that any large amount of rainwater floods the basements of the Federal Triangle buildings.
The cisterns are designed to help stop that. (Remember, the 2006 flooding in Commerce's basement left five feet of water down there, and it cost $15 million to repair it and the three other buildings that got flooded.)