Writing at The Smithsonian, Carl Abbott laments the tendency of political candidates to say that they intend to come to Washington, D.C. to "drain the swamp."
It's a phrase usually employed by outsiders and leveled against their opponents, presumably Washington insiders. If such people ever had good motives, they have been warped or stained by their time in the Washington swamp. Abbott says that the metaphor came about because of the long-held belief that the city was built on an actual swamp, which was translated into the figure of speech which we hear used today.
But the area that became the District of Columbia was never swampy, Abbott says, pointing out that George Washington, who selected it, had pretty significant experience as a land surveyor and would have been unlikely to select a poorly drained or situated site. Subsequent drainage or sewage problems stemmed from inadequate facilities for the removal of waste and excess water, rather than any inherently mucky quality of the ground. Abbott quotes several early 19th-century sources to demonstrate his thesis.
Which leaves us in the interesting state of acknowledging that labeling Washington, D. C. a swamp is inaccurate in a literal sense but dead on target as a figure of speech. In this case, the metaphor represents reality better than reality does.