Concord River - Dams Cause Fish Population To Decrease

Dams Cause Fish Population To Decrease

Dams were built along the Concord River to increase crop production and also to provide a source of power for operating mills. By the 19th century, the native population of shad and alewife became extinct, because the dams prevented the mature fish from returning upstream to spawn. Alewife and other anadromous fish are migratory. They hatch in fresh water, make their way to the sea to grow, then return as adults to fresh water to spawn, usually near where they had hatched. This instinct is imprinted within the fish when it is born. So, when the route upstream became blocked, the cycle was broken. The Millpond Dam in North Billerica is just one of many blockages that caused the alewife population to collapse on the Concord River. At this point, water was diverted north to Lowell and south to Charlestown to run the Middlesex Canal.