Boston Harbor's Water Clarity


A Secchi disk is used to measure water clarity in Boston Harbor

Water clarity is one way people decide whether to swim in Boston Harbor; it also determines how much light reaches the plants in the water. These plants, which include algae and seagrasses, are important to the ecology of the harbor.

Measurements of clarity in the harbor (and anecdotal observations by the public) indicate that water clarity has improved since the early '90s. One of the measurements that has shown this improvement is Secchi depth. This is the depth at which a white disk, a Secchi disk, lowered into the water is no longer visible.

Water clarity shows a west-to-east increasing gradient across the harbor. Water is more turbid toward the rivers and shallow margins of the harbor, and clearer (deeper Secchi depths) toward the mouth of the harbor and bay. Water in Dorchester Bay is naturally turbid, because it is a shallow depositional area for sediments carried down by the Neponset River.

Map of water clarity changes after South System flows were transferred to the Deer Island Treatment Plant for secondary treatment.