The Charles River is fed by approximately 80 streams and several major aquifers as it flows , starting at Teresa Road just north of Echo Lake () in Hopkinton, passing through 23 cities and towns in eastern Massachusetts before emptying into Boston Harbor. Thirty-three lakes and ponds and 35 municipalities are entirely or partially part of the Charles River drainage basin. Despite the river's length and relatively large drainage area (), its source is only from its mouth, and the river drops only from source to sea. The Charles River watershed contains more than of protected wetlands, referred to as Natural Valley Storage. These areas are important in preventing downstream flooding and providing natural habitats to native species.