 Marsh grass began growing in the flats, trapping the sewage and leading public health experts to conclude that the flats were unsanitary.Health officials also feared that the flats were a prime breeding ground for malaria- and yellow fever-carrying mosquitoes.By 1876, a large mudflat had formed just to the south of the western end of Benning Bridge, and another mudflat about wide had developed in the river south of that.