Treated effluent from sewage systems is usually diluted by lake water, but if the water becomes too shallow, there are serious risks of pollution from these outlets. Precipitation keeps the Great Lakes full. During the mild winter of 2000, snowfall and rain were unusually slight in most of the Great Lakes watershed. Spring came early to much of the region, allowing runoff to start early and providing warm weather to evaporate water in the tributaries as well as in the lakes themselves. At the same time, high temperatures drastically reduced ice cover on the lakes.
Ice normally covers large regions of the lakes in mid-winter, reducing evaporative loss of water.
The shallower lakes, Huron and Erie, showed the greatest change in lake levels, nearly 3 feet below long-term averages by early April. The deeper lakes, especially Superior and Michigan, dropped more slowly because their volume is so great relative to their surface area. These lakes were only a few inches below normal as of April. All lakes fluctuate from year to year, and water levels in the Great Lakes have changed a great deal since monitoring began more than a century ago. It is not clear whether these recent low levels are part of a serious trend.