Some water level fluctuations are short-term and are due to winds or changes in barometric pressure. These short-term fluctuations, usually lasting from a couple of hours to several days, can be very dramatic and due to storms or ice jams. Storm surge or wind-set up is when high winds from one direction can push the water level up at one end of the lake and make the level drop by a corresponding amount at the opposite end. When the wind subsides abruptly the water level will often oscillate back and forth like a pendulum until it stabilizes again. Similar to water sloshing back and forth in a bowl, this phenomenon is known as a seiche. Lake Erie is most susceptible to storm surges and seiches due to its east-west orientation in an area of prevailing westerly winds and its generally shallow western end.

Seasonal fluctuations range on average from 12 to 18 inches from winter lows to summer highs. The Great Lakes are generally at their lowest levels in the winter months. In the fall and early winter, when the air above the lakes is cold and dry and the lakes are relatively warm, evaporation is greatest. As the snow melts in the spring, runoff to the lakes increases.  Evaporation from the lakes is least in the spring and early summer when the air above the lakes is warm and moist and the lakes are cold. At times, condensation on the lake surface replaces evaporation. With more water entering the lakes than leaving, the water level rises. 

Long-term fluctuations occur over periods of consecutive years and have varied dramatically since water levels have been recorded for the Great Lakes. Continuous wet and cold years will cause water levels to rise. Conversely, consecutive warm and dry years will cause water levels to decline. Long-term fluctuations are shown on the hydrograph under Tracking Water Levels below. A hydrograph is a plot of water levels versus time. Global warming and a phenomenon known as the ‘greenhouse effect’ could cause significant changes in long-term lake levels. Although debatable, most predictions indicate that global warming would cause prolonged declines in average lake levels into the future. Besides natural climatic variability and potential man-made climate change, other factors can affect long-term fluctuations, including changes in consumptive use, channel dredging or encroachment and movement of the earth’s crust.