In Massachusetts, the Merrimack River planning basin includes the Stony Brook basin, the headwaters area of the Souhegan River basin, and the drainage area of the Merrimack River below the confluence of the Merrimack and Concord Rivers, excluding the Shawsheen River basin. The basin, sometimes referred to as the lower Merrimack River basin, is located in Essex and Middlesex Counties and includes all or part of 28 cities and towns, including the cities of Haverhill, Lawrence, and Lowell.

Altitudes in the basin range from sea level to about 300 ft above sea level at the tops of small hills. Unless otherwise noted, the following description of the water resources of the Merrimack River basin is based on Hydrologic Investigations Atlas 616 (Gay and Delaney, 1980b).