Surface water

The Merrimack River basin covers about 279 mi2 of northeastern Massachusetts. Within this area, there are 76 lakes and ponds, 42 of which are 10 acres or larger. Only one lake, Lake Cochichewick in North Andover (592 acres), is larger than 500 acres. The Merrimack River flows in a well-defined channel that ranges in width from about 400 to 1,400 ft. About 9 mi of the river above its mouth at Newburyport are part of an estuary bordered by tidal marshes. From the State line to the Merrimack estuary, a distance of about 50 river mi, the river drops about 90 ft in altitude. This drop includes flow over dams in the cities of Lowell and Lawrence, where some streamflow is diverted through extensive canal systems.

Streamflow in the Merrimack River is affected by reservoirs, especially those used for flood control and water supply. Flood-control reservoirs store water during peak streamflow and release it during periods of lower streamflow. From 1960 through 1974, water diverted by the city of Worcester and by the MDC (now the MWRA) from the tributary Nashua and Concord River basins by way of Pine Hill, Kendall, Sudbury, and Wachusett Reservoirs reduced the mean annual streamflow in the lower Merrimack River by about 177 ft3/s. Although the Merrimack River flows through or adjoins 15 municipalities in the basin, only Lowell, Lawrence, and Methuen use it for water supply.

Hydroelectric power production along the Merrimack River and its tributaries causes diurnal streamflow fluctuations in the lower part of the basin, especially from July to September, when streamflow contributions from tributary drainage areas are small.