Lake retention time (also called the residence time of lake water, or the water age or flushing time) is the calculated average time that water (or a dissolved substance) spends in a lake. At its simplest this figure is the result of dividing the lake volume by the flow in or out of the lake. It describes the amount of time that is needed for a substance to flow in and out of a lake.
Lake retention time Facts for Kids. Kiddle Encyclopedia.