The Mousam River

The Mousam River watershed contains 349 miles of river and streams and drains a total area of about 117 square miles. It covers portions of the towns of Acton, Shapleigh, Waterboro, Sanford, Alfred, Lyman, and Kennebunk. The upper portions of the watershed are dominated by forests, wetlands, and agricultural lands. Development here is focused on the shorelines of numerous lakes and ponds, including Square Lake, Mousam Lake, Shaker Pond, and Bunganut Pond, and on the mainstem of the river as it passes through downtown Sanford. The  mainstem meets the Littlefield River and the Middle Branch of the Mousam at Estes Lake, and from there the river flows toward downtown Kennebunk. Just below Route 1 in downtown Kennebunk, the river meets the head-of-tide and then flows through a rich estuary and tidal salt marshes, much of which is within the Rachel Carson National Wildlife Refuge.


The Mousam watershed is a rich and diverse landscape with many areas considered to be of “statewide ecological significance” by the State of Maine. The Massabesic Forest, the Kennebunk Plains, and several others areas contain rare and exemplary natural communities that support a broad diversity of unique plants, animals, and wildlife. The Mousam River and its hundreds of miles of tributary streams once provided a seamless connection between these “upland” areas and the estuary and ocean, a link that existed for millennia and provided benefits to both the freshwater and marine environments. This great connection is perhaps best exemplified by the large runs of sea-run that returned to the Mousam every year. Species like American shad, alewives, blueback herring, American eel, and Atlantic salmon constituted a huge amount of biomass and many of these species brought with them a large influx of marine-derived nutrients that benefitted the terrestrial and riparian communities. The construction of dams on the Mousam broke this great natural cycle and caused the near loss of the river’s native sea-run fisheries and greatly impacted the fish and wildlife that depend upon them.