The dam controls the surface level of the river basin as well its tributaries upstream, including the Back Bay Fens and Muddy River and to prevent sea water from entering the Charles River freshwater basin during high tides. It replaced the 1910 Charles River Dam upstream, now the site of the Boston Museum of Science . The 1910 dam includes two locks (one under the Science Museum parking garage). Both locks are now kept open for navigation. The older dam could not keep sea water out and a layer of salt water accumulated at the bottom of the fresh water basin, contributing to pollution and fish migration problems.
