The Tidal Basin is a partially man-made reservoir found in the Capital Wasteland.
Before the War, it was a productive fishing and boating port with the Jefferson Memorial in its center. When the bombs fell, mirelurks began colonizing it, soon infesting the water. The basin is filled with many row boats, tugboats, and dinghies. Many boxes of loot are still aboard the half-sunken, and dilapidated boats that mar in the shallows of the district's port basin.
It's surrounding territory was being fought over by the Brotherhood of Steel, The Enclave, and the super mutant hordes that occupied Downtown Washington, D.C., and in 2277 the Brotherhood seized it and gained control of Project Purity. The Tidal Basin could have been infected with the modified FEV if the player chose to do so during the final part of Take it Back! main quest.
The Tidal Basin appears only in Fallout 3.