Surface water

The Mystic River basin has an area of about 66 mi2 in northeastern Massachusetts. The basin has small hills, which reach altitudes of about 350 ft above sea level, and many wetlands located in broad valleys. The low relief of the land has caused many wetlands to form throughout the basin, and many lakes, ponds, and swamps exist along the valleys of the Mystic and Aberjona Rivers. The basin contains 44 lakes and ponds, 20 of which have an area of 10 acres or more. The largest lake in the basin is Spot Pond in Stoneham, which is 307 acres in area.

The Mystic River begins at the outlet of Lower Mystic Lake and flows southeastward to Boston Harbor. Major tributaries are the Aberjona River, which begins in Reading, and Mill Brook, which begins in Lexington. The entire length of the Mystic River was originally influenced by tides. However, in 1909, Craddock Locks (now abandoned), located in Medford, prevented tides from affecting the upper part of the Mystic River, Lower Mystic Lake, and Alewife Brook.

Since 1966, the Amelia Earhart Dam, located at the mouth of the Mystic River, has prevented tides from affecting the Malden River and the lower part of the Mystic River. Stream gradients in the basin are generally low. For example, the Aberjona and Mystic River system loses about 80 ft of altitude in 16 mi from the headwaters in Reading to the Amelia Earhart Dam near Boston Harbor, an average stream gradient of 5 ft/mi.

Flow in the Aberjona River commonly is reduced by groundwater withdrawals along the river and subsequent diversion out of the basin by way of the MWRA sewer system. This reduction is most noticeable during low-flow periods. For example, compared to nearby river basins such as the Assabet, Charles, and Shawsheen, the annual 7-day, 10-year low flows in the Aberjona River at Winchester are about one-half to one-third of their expected value.