The East River watershed covers half of Guilford and one third of Madison, and all water within it flows downhill through a network of streams into the East River and then into Long Island Sound. Pollution from the watershed also passes through these marshes, which act as a natural biological filter. Between 1880 and 1970, about 30 percent of Connecticut's tidal marshlands were lost to development, with a decline in species and water quality in rivers and Long Island Sound. The East River retained almost all of its original tidal marshes because they were donated to conservation groups in the 1960s. Anne Conover began this effort by donating these marshes to Audubon in 1964, and she also helped bring about Connecticut's Tidal Wetlands Act of 1969, which ended the dredging and filling of the state's tidal marshes.