The Mousam River Dams



Ecological impacts of dams include...

- BLOCKING upstream and downstream movement of fish

- ALTERING water temperatures

- BLOCKING and slowing river flows

- ALTERING the natural timing of flows

- OBSTRUCTING the movement of nutrients and sediment

The ecological Impact of the Dams on the Mousam River


There are 15 known dams in the Mousam watershed, including 11 on the mainstem between the outlet of Mousam Lake in Shapleigh and the head-of-tide in Kennebunk, a stretch of river only 24 miles long. This large number of dams over a relatively small stretch of river makes the Mousam one of the most heavily dammed rivers in the State of Maine. In fact, out of the over 40 major river systems in Maine, the Mousam is the only one completely lacking fish passage.

The health of the Mousam River has suffered tremendously due to a long history of degradation from pollution and dams. While great progress has been made in addressing the most severe water issues in the watershed, numerous problems still persist. As of 2012 the estuarine portion of the Mousam River is in non-attainment for dissolved oxygen (DO) and shellfish beds in the estuary are closed due to concerns about bacterial contamination. Little information is known about the water quality in impoundments in the lower river and the majority of dams in the watershed have no minimum flow requirements, and those that do are either “voluntary” or are incredibly low and not protective of aquatic habitat.

While dams can completely alter and degrade a whole suite of natural biological, chemical, physical, and ecological processes, nowhere are the impacts more pronounced than on fish communities and populations. For millennia, the Mousam River was home to large migrations of native sea-run fish. Every year, runs of Atlantic salmon, American shad, alewife, blueback herring, American eel, sea lamprey, tomcod, sea-run brook trout and other species migrated into the Mousam. Species like Atlantic salmon and blueback herring would spawn in the riffles and faster moving water of the river, while alewives would travel into the watershed’s lakes and ponds to reproduce. Each of these species was incredibly important to the overall ecology of the watershed, bringing important nutrients into the freshwater environment and serving as a food source for other fish, birds, and wildlife. These fish were also important economic and cultural resources for the human populations in the watershed, providing both sustenance and extremely valuable commercial fisheries.

Commercial fisheries once existed for tom cod, the dimunitive cousins of the larger Atlantic cod, and for striped bass and shad. The runs of shad, river herring, and salmon were so plentiful that local residents would fill wheel barrows with their catch with minimal effort. However, since the re-construction of dams on the lower Mousam about a century ago, there has been no fish passage on the river. The great runs of these sea-run fish have been almost completely eradicated. Remarkably, despite being relegated to about 1% of of their historic spawning habitat in the Mousam, alewives, blueback herring, shad, and the occasional salmon still return to the river every spring.

With the selective removal of some dams and the construction of fish passage at others, it would be possible to rejuvenate runs of river herring and shad in the Mousam. Providing these species with access to Estes Lake, the Middle Branch, and Littlefield River portions of the watershed could result in the annual runs of 56,000 American shad, 340,000 blueback herring, and 223,000 alewives. Furthermore, eliminating unhealthy impoundments and restoring portions of the river to a natural, free-flowing condition could provide more than 20 miles of habitat for resident and sea-run brook trout, which would support a remarkable recreational fishery for coastal southern Maine.

Learn More about the Mousam River Dams