Special Report:
Great Lakes Levels Mixed at Mid Year

In a report issued in July by the University of Wisconsin Sea Grant Institute, water levels in lakes Michigan, Huron, and St. Clair peaked at levels lower than levels recorded last summer. Levels of Lakes Erie and Ontario rose to slightly above average at mid-year, and well above average, respectively, thanks to heavy June rainfall. Only on Lake Superior are water levels expected to continue rising this summer. The present situation on each lake is given below. 

The lower-than-average water level condition has apparently ended (for the time being) on Lakes Erie and Ontario with surprising mid-year rises due to heavy June rains. Greater than average rainfall in June also benefited Lake Superior, and probably Lake St. Clair. Lake Superior is expected to continue rising, Michigan and Huron are believed to be at their seasonal peak level and the other lakes are believed to have declining water levels. The present lake level situation and the new six month forecast from the U.S. Army Corps of Engineers indicates what to expect for the busy fall shipping season on the lakes. 

According to the Corps of Engineers' new July 2000 Monthly Bulletin of Lake Levels for the Great Lakes, Lakes Superior and Erie are expected to begin September several inches lower than last year. Lakes Michigan, Huron and St. Clair are expected to begin the month about half a foot lower than last year. Lake Ontario is expected to be 10 inches higher than last year at the beginning of September. By the middle of December, all of the lakes are expected to be within two inches of last year's lake levels. 

Precipitation figures from the U.S. Army Corps of Engineers indicate that the individual lake basins of the Great Lakes had average to above average precipitation for the past 12 months through June 2000. Heavy rains in certain months (like May and June) made up for dry months and lack of snowfall in the water budget. Evaporation has increasingly been blamed for much of the drop in lake levels. 

Warmer-than-average water temperatures in the past few summers caused greater than average evaporation of lake water when the cold winds of fall arrived. Brisk, dry, cold winds blowing over exposed warmer waters hastens evaporation and lowers water levels. According to the Corps of Engineers (Detroit District), Lake Michigan loses about 2.5 centimeters (one inch) of water a week to evaporation in October. If an unusually cold air mass settles over a much warmer lake, the same amount of evaporation, or more, may occur in just a few days. Since lakes Michigan and Huron rise and fall together, a drop of one inch in water level is a loss of about 784 billion gallons (0.784 trillion gallons) of water from these two lakes to the atmosphere. 

In certain Great Lakes harbors during times of low lake levels, cargo-loading ships are vulnerable to being set down on the harbor bottom while loading if the harbor water experiences a temporary drop in water level caused by a atmospheric pressure change, or a shift in wind direction to strong winds blowing offshore. On June 2nd, Algoma Central Marine's vessel, Algowood buckled while loading stone at the dock in Bruce Mines, Ontario. Bruce Mines is a small port in the northwest end of the North Channel of Lake Huron. The vessel hull "hogged" when the midsection settled to the bottom in about 24 feet of water, with bow and stern on the bottom in 26 feet of water.

Lake Superior

Lake Superior rose dramatically in June (six inches) and maintained a level through the first two weeks of July that is five inches above chart datum (also known as Low Water Datum or LWD) of 601.1 feet, International Great Lakes Datum 1985 (IGLD 1985). Precipitation in the Lake Superior Basin was 15 percent above average in June and 11 percent above average for the past 12 months. The mid-July level is four inches lower than the lake level one year ago, eight inches below the long term average level for this date, but 15 inches above the lowest average July lake level in this century, a record set in 1926.

The lake is expected to rise two inches by mid-August, peak at another inch higher in September (two inches lower than last year), and decline to four inches above chart datum by mid-December. Fall lake levels are expected to "improve" slightly during the fall shipping season, relative to last year. September is expected to begin with a lake level two inches lower than last year, and the mid-December level is expected to be two inches higher than the mid-December level last year, but five inches below average.

Lakes Michigan and Huron

Lakes Michigan and Huron share a common water level through the broad, deep Straits of Mackinac. The water level rose three inches in June and one more inch in the first two weeks of July: the least of the rises in the Great Lakes. Precipitation in the basin of these lakes was four percent below average in June and one percent above average for the past 12 months.

On July 14th, the water level in these lakes was five inches above the chart datum of 577.5 feet (IGLD 1985). This level is 20 inches below the long-term average lake level for the date, 10 inches lower than the level one year earlier, but 15 inches above the lowest average July level in this century, a record set in 1964.

Both lakes are expected to remain at this level through mid-August. The total seasonal water level rise this year appears to be ten inches, slightly below the average 11-12 inches seasonal rise, if the present level is the seasonal peak level this year. The July forecast indicates most likely fall lake levels "improving" relative to last year with September beginning at five inches lower than last year and a most likely water level in mid-December that will be about the same as last year.

Lake St. Clair

This lake rose seven inches in June and declined an inch by mid-July. On July 14th, the lake level is 24 inches above the chart datum of 572.3 feet (IGLD 1985). This level is seven inches below the long-term average lake level for the date, just four inches lower than the level on the same date last year, but 22 inches higher than the lowest average level in July, (a record set in 1934).

Lake St. Clair is expected to decline four inches by mid-August 30th. This lake rose 23 inches from mid-February to the end of June: a seasonal rise much greater than the average seasonal rise of 16 inches. Water level changes on this lake can be quite dramatic in response to hydrologic conditions in the upstream lakes. Water levels this fall are expected to "improve" relative to last year; beginning September at six inches lower than last year and reaching last year's level in mid-December.

Lake Erie

Lake Erie's water level rose nine inches in June, then declined three inches in the following two weeks. On July 14th, Lake Erie's level is 31 inches above the chart datum of 569.2 feet (IGLD 1985). This level is one inch below the long-term average level for the date, two inches higher than the water level on the same date last year, and 33 inches higher than the lowest average level in July, a record set in 1934. From mid-February to the end of June, Lake Erie rose two feet, nearly double the long-term average seasonal rise of 13 inches. Precipitation in the Lake Erie Basin was 54 percent above average in June and one percent below average for the past 12 months: the only Great Lakes sub-basin with below average precipitation for the past 12 months.

Lake Erie is expected to decline four inches by mid-August. Lake levels for September through mid-December are expected to be two inches lower than last year's fall lake levels.

Lake Ontario

Lake Ontario's water level rose four inches in June and declined three inches in the following two weeks. On July 14th, Lake Ontario is 44 inches above the chart datum of 243.3 feet (IGLD 1985). This level is seven inches above the long-term average lake level for the date, 19 inches higher than the level one year ago, and 44 inches than the lowest average level in July, a record set in 1935. The Lake Ontario Basin had 51 percent above average precipitation in June and five percent above average precipitation for the past 12 months.

Lake Ontario is expected to decline six inches by mid-August. Lake levels for September are expected to begin 10 inches higher than last year and decline to average levels; two inches higher than last year, in mid-December.
This report was written by Phil Keillor, Coastal Erosion/Engineering, UW Sea Grant Institute and copyrighted by UW Sea Grant Institute. The UW Sea Grant Institute is headquartered at the University of Wisconsin-Madison. It is part of a national network of 30 university-based programs of research, outreach and education dedicated to the protection and sustainable use of the United States' coastal, ocean and Great Lakes resources.

The National Sea Grant Network is a partnership of participating coastal states, private industry and the National Sea Grant College Program, National Oceanic & Atmospheric Administration and the U.S. Department of Commerce.

For information about this program, visit www.seagrant.wisc.edu.