Does it ever not rain in that town?
It's our sub-tropical weather pattern. In the 1800s, folks left town because of the posibility of malaria eminating from the canal which is now Constitution AVE. DC is often referred to as a swamp. Much of West Potomac Park, Lincoln Memorial area, National Airport, etc. were a swamp at one time. There is a marker on the Washington Monument grounds where boats used to dock. Many of the monuments, etc. are built on landfill. In essence, the swamp will never be drained. It will only be backfilled with rubble.