ASK TOM: Lake Michigan's "flushing time"

Dear Tom,

Considering the water that flows into or falls into Lake Michigan and the water that flows out, how long does it take for the water in Lake Michigan to recharge?
Dan Correli, Chicago  
Dear Dan,
It’s a complex question and the answer is, at best, an estimate. Hydrologists define the “flushing time” of a lake as the time required for the average outflow from a lake to drain it; the volume of a lake divided by the average rate of outflow. Lake Michigan’s flushing time is estimated to be 69.5 years. Permanent lakes with a flushing time less than a year are rare. They tend to have vigorous flow and their biological diversity is low because there is little time for nutrients to accumulate in the water. At the other extreme, lakes with flushing times greater than ten years are sensitive to external changes, such as pollution. At least one flushing is required to recover from pollution.