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Some history:

When President George Washington and city planner Pierre Charles L’Enfant selected the site of the new national capital in 1791, the land was a mix of forested hills, bluffs, crop land, and waterways. The tidal flats of the Mall lay between the Potomac River and Tiber Creek, which flowed along today’s Constitution Avenue, entering the Potomac between marshy shores near the present-day Washington Monument. This modern painting shows how the banks of the Tiber may have looked in the early 1800s, when herons, kingfishers, wild ducks, and small water animals lived on its banks.
Diary, John Quincy Adams, 1825

In the early 1800s, early Washington area residents hunted, boated, fished, and swam in Tiber Creek. This diary entry from the 1820s describes a day when President John Quincy Adams, a strong swimmer, nearly drowned in the Tiber. Visitors to the city expected to see a beautiful national capital and were not pleased when the Tiber overflowed during heavy rains creating mud puddles and soggy streets around the Mall.
West front of Capitol, vii, peristyle complete

As the city developed, erosion and silt deposits filled the natural drainage channels on the National Mall. Sewage and garbage clogged the Washington City Canal, built to channel the waters of Tiber Creek. This photograph shows the puddling of the Creek on the western grounds of the Capitol in 1860. Mosquitos and other disease-bearing insects bred in these deep puddles and marshy areas. The Mall appeared less like the beautiful parkland its planners envisioned and more like a swamp.

The 1870s marked a change in the waterlogged landscape of the National Mall. The city filled in the Washington Canal and covered Tiber Creek, which now flows beneath Constitution Avenue. The Army Corps of Engineers began dredging the Potomac River and worked for several decades to clean and widen the river channel. The material dug from the Potomac became landfill on the National Mall, adding more than 700 acres to the landscape and filling in marshy areas. This photograph was taken at the very beginning of the work, when the areas beyond the Washington Monument was still tidal flats.
Levees on the National Mall

Today, swamp-like, marshy pockets no longer dot the National Mall and levees protect it from flooding. The first levees were built in the 1930s after a bad flood. They are hidden in the landscape by walkways and gentle slopes between the Vietnam Veterans’ Memorial and the Reflecting Pool and from the Washington Monument to the Lincoln Memorial. In 2006, 3 feet of storm water covered the Mall and flooded museum and office building basements on Constitution Avenue. Congress then authorized new levees be built on the National Mall at 17th Street. These levees can withstand 700,000 cubic feet of water pressure per second, and will help to prevent massive flooding on Mall.