 Between 1860 and the late 1880s, large mudflats ("the Anacostia flats") formed on both banks of the Anacostia River due to this deforestation and runoff.At this time, the city allowed its sewage to pour untreated into the Anacostia.Marsh grass began growing in the flats, trapping the sewage and leading public health experts to conclude that the flats were unsanitary.