Water typically remains in Lake Superior for about 191 years before leaving the lake.
This figure is called the lake’s retention time, or residence time. It is an average calculated from the lake’s volume and the rate at which water enters and exits; individual water molecules may remain for shorter or longer periods.
Lake Superior’s exceptionally long retention time reflects its enormous volume and relatively modest outflow through the St. Marys River. By comparison, shallower or smaller Great Lakes exchange their water much more quickly.
Retention time is important in environmental science because pollutants can persist in a large lake for decades or centuries. It does not mean the entire lake remains unchanged: currents, seasonal mixing, evaporation, precipitation, and tributary inflow continually redistribute water.