The Charles River is the longest river that remains entirely within Massachusetts, stretching more than 80 miles from its headwaters near Echo Lake in Hopkinton to Boston Harbor where it divides Cambridge and Boston. Throughout its journey, the river is fed by approximately 80 streams and aquifers, flowing through 23 cities and towns. Despite its impressive length, the river only drops 350 feet in elevation from source to mouth. The Charles River's watershed encompasses more than 8,000 acres of wetlands, which serve as natural flood control and support a diverse ecosystem. Once heavily polluted due to Boston's rapid development in the early 1600s, significant restoration efforts by organizations like the Environmental Protection Agency have successfully improved the river's health, allowing a diverse ecosystem to thrive once again.