Water that is in a lake does not stay in that lake. Where does the water go? The length of time that it takes for the amount of water in a lake to be completely replaced (enter the lake) is called retention time. Each of the Great Lakes has a different retention time. Construct an appropriate model of the water flow of the Great Lakes learn about retention time and replacement time, and how how Lake Superior affects the dynamics of water flow, retention time and flushing rates for the Great Lakes system.