Lake levels can change over periods of years for the same reasons. During periods of years in which precipitation and runoff in the Great Lakes basin are high and evaporation low, lake levels can gradually increase. In periods of low precipitation and high evaporation, lake levels can gradually lower. The resultant variation in lake outflows offsets a part of these water supply variations, but not all, hence the variations in lake levels. The length of time required for noticeable changes, and the degree of the changes, will depend on how wet or how dry the weather is and on ambient temperatures.
Water levels can change in a matter of hours.
When the wind stops, the water will oscillate back and forth until it levels itself out, much as it would in a bathtub. Since long-term lake level fluctuations are influenced by weather trends, we cannot predict when and how often extremes in levels will occur. Extremely high levels occurred in the early 1950s, the early 1970s, and the mid-1980s. The most recent record high lake levels period occurred between 1985 and 1987 when all of the lakes, except Lake Ontario, reached their highest levels recorded in the twentieth century. Over the following two years, lake levels dropped rapidly to their long-term averages.