Lower Mystic Lake is a
meromictic lake, which means that the lake has a deep layer of water that rarely, if ever, mixes with its top waters. As a consequence, the sediments at the bottom of Lower Mystic Lake accumulate in annual layers (or
varves) that have been nearly undisturbed for a thousand years. Such varves in meromictic lakes preserve a historical record somewhat like tree rings do. In the case of Lower Mystic Lake, the varves have been used by Mark Besonen and his collaborators to study the historical incidence of hurricanes. Although the Mystic Lakes are popular for swimming, sailing, and fishing, the Upper Mystic Lake suffers from contamination by
arsenic and other heavy metals from the
Aberjona River. The eastern shore of the lakes is part of the
Mystic River Reservation managed by the Massachusetts
Department of Conservation and Recreation. The lakes were previously part of the drinking water supply for Charlestown and later Boston (see
Massachusetts Water Resources Authority). ==History==