The Quinnipiac and Mill River salt marsh was once the largest salt marsh in the state, but gradual filling left the 1000-acre Quinnipiac Marsh. Railroad bridges built across the marsh in the late 1800s decreased tidal action in the upstream marshes and reduced their salinity, changing the dominant marsh grass from salt hay to cattails, and now Phragmites has encroached on much of the marsh. In the 1930s, ditches dug for mosquito control later became nurseries for fish, crabs, and other invertebrates, and some small fish now feed on mosquito larvae. The Quinnipiac River was first declared polluted in 1914 after businesses and municipalities dumped untreated wastewater into it, and although state, federal, and local action greatly improved river health, pollution still comes from malfunctioning controls during low water flow, embedded contaminants, landfill leachate, the tire pond, and storm runoff. Human changes to the marsh also altered wildlife: muskrats expanded after cattails replaced salt grasses, then declined sharply between 1970 and 1990, likely because Phragmites supplanted cattails, while osprey nesting platforms helped bring ospreys back, with fledging numbers rising from one in 1996 to twenty-two in 2003. Preservation of the marsh's biodiversity and improvement of public access continue through the work of several governmental, nonprofit, and volunteer organizations.