Although the annual changes in water level in Green Bay can vary greatly in any year, the average annual cycle over the last 80+ years is approximately 12 inches.

Water level depends primarily on a balance between water input from precipitation on the lake and its drainage basin, and evaporation and transpiration (plants greatly increase the rate of water loss). In winter precipitation is low and primarily in the form of snow which remains on the land surface. In spring the bay receives a large input of meltwater from the watershed and the amount of monthly precipitation increases. By mid summer the runoff contribution from the watershed has diminished and warm, sunny (and windy) days contribute to falling water level.

The above discussion refers to averages, however. As seen below the point at which water levels reach minimum or maximum in any given year can vary.