Book 30, Question 84
Why do the Great Lakes rise and fall from year to year?
Lake levels go up and down with the balance between water coming in and water going out.
Rain, snow, melting snow, and rivers add water, while evaporation and outflow through rivers remove it. A wet few years can raise a lake, and a dry, warm few years can lower it. Levels also change with the seasons. Shoreline homes and harbors have to plan for these swings.