Although part of a single system, each lake is different. In volume, Lake Superior is the largest. It is also the deepest and coldest of the five. Superior could contain all the other Great Lakes and three more Lake Eries. Because of its size, Superior has a retention time of 191 years.
Retention time is a measure based on the volume of water in the lake and the mean rate of outflow.
Most of the Superior basin is forested, with little agriculture because of a cool climate and poor soils. The forests and sparse population result in relatively few pollutants entering Lake Superior, except through airborne transport. The northern region of the Great Lakes is sparsely populated and is characterized by coniferous forest and rocky shorelines. Above, the western shore of Georgian Bay in the Bruce Peninsula National Park. Lake Michigan, the second largest, is the only Great Lake entirely within the United States.