Fluctuating water levels

The Great Lakes receive their supplies of water from inflows from upper lakes in the chain and from precipitation that falls not only on the lakes themselves but also on their drainage basins and eventually flows into the lakes. They lose water through evaporation, outflows, and consumptive uses. The difference between the amount of water coming into the lakes and the amount going out determines whether lake levels remain steady, whether they rise, or whether they fall.

Because the combined effects of precipitation, runoff, and evaporation vary from season to season and from year to year, the levels of the lakes also vary. Lake outflows also vary as a function of lake levels. For example, the lakes usually rise in the spring due to additional runoff and recede in late summer and early fall as runoff decreases.

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.

Since long-term lake level fluctuations are influenced by weather trends, we cannot predict when and how often extremes in levels will occur. Levels of some of the Great Lakes fell to record lows in the late 1920s, the mid-1930s, and the mid-1960s. 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. The 1990s were characterized by persistently high water levels and outflow throughout the Great Lakes-St. Lawrence system.

Water levels can also change in a matter of hours. Sustained high winds from one direction can push the water level up at one end of the lake (this is known as "surge") and make the level go down by a corresponding amount at the opposite end. When the wind stops, the water will oscillate back and forth until it levels itself out, much as it would in a bathtub. This is known as "seiche".

The effects of water level fluctuations on human activity

For people living on the shores of the Great Lakes, high water levels can increase the risks of shoreline flooding, erosion, and damage caused by waves during storms. Industries and commercial operations along the lake shores can face the same risks. While high water levels allow ships to carry heavier loads, extremely high levels and flows through the connecting channels can cause navigation problems. High levels can also be beneficial for hydroelectric plants, which can produce more electricity with the additional water. During extremely high water levels, however, the amount of water available to these plants can exceed their capacity and be spilled.

Erosion of the shoreline at one point will supply the sand that helps build a beach farther down shore. This contributes to the constantly changing shorelines of the Great Lakes. Low water levels increase the size of lakeshore beaches, but extremely low levels can expose unsightly and, sometimes, hazardous rocks, mudflats, and other objects that can pose problems to both swimmers and boaters. As well, very low water levels can make some recreational docking facilities unfit for use. For industries depending on ships to transport their products and supplies, extremely low levels can interfere with loading and unloading. These ships may also have to carry lighter loads. Very low levels may cause reduced flows through connecting channels and thereby result in reduced hydropower production.

The effects of human activity on lake levels

Several human activities have affected levels and flows of the Great Lakes. For example, structures have been built to regulate the outflows of Lakes Superior and Ontario. Lake Superior has been regulated since 1921 as a result of hydroelectric and navigation developments in the St. Marys River. Lake Ontario has been regulated since 1960 after completion of the St. Lawrence Seaway and Power Project. Besides assisting navigation and allowing for dependable hydropower, these regulation structures have helped, to some extent, to stabilize the range of lake level fluctuations.

Diversions bring water into, and take water out of, the Great Lakes. The Long Lac and Ogoki diversions bring water into Lake Superior from sources that once flowed into James Bay. They were constructed for hydropower generation and logging. The Lake Michigan diversion at Chicago takes water out of Lake Michigan to the Mississippi River for domestic, navigation, hydroelectric, and sanitation purposes. The Welland Ship Canal, which was built to allow ships to bypass Niagara Falls and to provide water for power generation, routes additional water out of Lake Erie into Lake Ontario.

In addition, the St. Clair and Detroit rivers have been dredged and modified. This has caused some drop in the levels of Lakes Michigan and Huron. Channel and shoreline modifications in connecting channels of the Great Lakes have affected lake levels and flows as well. For example, in the Niagara River, construction of bridges and infilling of shoreline areas have slightly reduced the flow carrying capacity of the river. 

Since early in the twentieth century, significant changes in land use in the Great Lakes basin have occurred, including deforestation, urbanization, and drainage of wetlands. These activities have changed the runoff characteristics of the drainage basin. Although the extent to which these changes affect lake levels is difficult to define, research suggests that land use changes have increased water flows into the Great Lakes from some tributary streams.

Human effects on lake levels have been relatively small, however, compared to the changes caused by the natural factors described earlier.

Resolving concerns about fluctuating Great Lakes water levels

The Governments of Canada and the United States provide the mandate and technical support to the International Joint Commission (IJC) for their international operational boards that monitor and decide on flow conditions in the St. Marys, Niagara, and St. Lawrence rivers. The IJC also carries out major reference studies on Great Lakes levels. The most recent investigation was completed in 1993 in response to a request by the Canadian and United States governments in 1986, when Great Lakes water levels were at record highs for the twentieth century.

This IJC reference study examined all lake level interests in the basin, including owners of shoreline property, fishers, boaters, shippers, wildlife, and producers of hydroelectricity. Among the findings was the recommendation that federal governments should not undertake commitments to further regulate Great Lakes levels as a means of reducing shoreline flooding and erosion on the lakes. Instead, comprehensive shoreline land use and management programs should be undertaken to help adapt shoreline activities to fluctuations in water levels.

The Government of Canada, through Environment Canada, also established the Great Lakes-St. Lawrence Water Level Information Office in Burlington, Ontario to act as a focal point for material and public contact regarding fluctuating Great Lakes water levels. In times of extreme high or low water levels, Environment Canada, in cooperation with provincial agencies, is instrumental in providing information and warnings of events to the many Great Lakes interests affected by lake levels. Environment Canada has also cooperated with and provided funds to the province of Ontario and local conservation authorities to identify, map, and plan for more effective use of shoreline lands that are prone to flooding and erosion hazards.

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