The Merrimack is the second largest surface drinking water source in New England, serving more than 300,000 people. The communities of Lowell, Lawrence, and Methuen utilize the river as its drinking water supply.

• Precipitation
• Runoff
• Stream Flow

Precipitation

The river receives an annual average of 43 inches of precipitation with the northern areas averaging slightly more (46 inches) and the southern areas slightly less (41 inches). Although precipitation is evenly distributed throughout the year, the water in snowfall is stored through the winter months for release during spring thaw.

Runoff

During March, April, and May, rainfall combines with spring thaw to release 50% of the basin's average annual runoff.
Statistics from 1978 state the average annual runoff to be 21 inches, representing about 51% of annual precipitation. These numbers have most likely increased over time due to development associated with impervious surface cover. The balance is returned to the atmosphere by direct evaporation or transpiration from vegetation.

Stream Flow

Stream flow in the Merrimack River is affected by reservoirs, especially those used for flood control and water supply. Flood control reservoirs store water during peak stream flow and release it during periods of lower stream flow.
Stream flow is also impacted by the use of the Merrimack for power. The Merrimack is still used to supply power via six major hydroelectric dams on the river and almost 100 small power projects. Hydroelectric power production along the Merrimack and its tributaries causes diurnal stream flow fluctuations in the lower part of the basin, especially from July to September, when stream flow contributions from tributary drainage areas are small.
Another factor attributing to stream flow is temperature, which influences the rate of evaporation and transpiration. The temperature in the region varies at an annual average between 43̊F and 50̊F. Figure 2 provides a clear illustration of how each of these factors contributes to stream flow.