- Long-term water levels in the Great Lakes have fallen since reaching record highs in the 1980s.
- Warmer temperatures and higher evaporation rates are partially responsible for historical declines in lake levels.
- Land use and lake regulations also affect lake levels, though no major management changes have occurred since 2000.
- Models disagree on the long-term future of lake levels. Periods of both high and low water levels should be anticipated.
Great Lakes water levels are affected by climate change, resource management, and natural climate variability. Models disagree on the direction of future water levels. Many studies project that long-term declines in lake levels will continue. More recent studies that better account for evaporation project that lake levels will likely rise. Regardless, seasonal, annual, and multi-year variability in lakes levels will remain large. Prolonged periods of both high and low water levels should be anticipated.
Evaporation, Precipitation, and Lake Levels
Overall, long-term water levels in the Great Lakes have fallen since record highs were reached in the 1980s, and changes in precipitation and evaporation may have partially contributed to declines since that time.1 Increasing precipitation can add to the to water supply of the basin and drive lake levels higher, but precipitation competes with evaporation, which depends on ice cover. air, and water temperatures. In the absence of other non-climatic factors, if precipitation outpaces evaporation, lake levels are expected to rise. If it is the reverse, and evaporation exceeds precipitation, lake levels are expected to decline.
With warmer air and water temperatures, summertime evaporation rates have more than doubled since the 1980s.2 3 4 Warmer water temperatures also reduces ice cover on the lakes, allowing for evaporation from exposed surface water throughout a longer part of the year.5 Recent studies have employed new methods for measuring evapotranspiration from the Great Lakes and have projected a drop in lake levels smaller than previously predicted by other studies, or even reversed in some cases, with lake levels rising.6
Non-Climatic Factors in Lake Level Decline
The dredging of the St. Clair and Detroit Rivers prior to 1962 are estimated to have lowered Lake Michigan-Huron's baseline level by 16 inches.1 There have been no major dredging projects in recent years, however, so it is unlikely that dredging has contributed to lake level changes in recent decades.
Erosion in parts of the St. Clair River since 1962 may have lowered water levels by an additional 3 to 5 inches, but the river has not seen significant erosion since 2000.1
The Chicago River was reversed to divert water from Lake Michigan to Chicago starting in 1900. The process is estimated to have permanently lowered basin-wide water levels by about 2 inches, but this has been compensated by the Long Lac and Ogoki diversions into Lake Superior during the 1940s.1
- 1. a. b. c. d. IUGLS, 2009. Impacts on Upper Great Lakes Water Levels: St. Clair River. Final Report to the International Joint Commission.
- 2. Elizabeth A. Pendleton, E. Robert Thieler, and S. Jeffress Williams (2010) Importance of Coastal Change Variables in Determining Vulnerability to Sea- and Lake-Level Change. Journal of Coastal Research: Volume 26, Issue 1: pp. 176 – 183
- 3. Lofgren, B.M., Quinn, F.H., Clites, A.H., Assel, R.A., Eberhardt, A.J. (2000). Water Resources, in Sousounis, P.J., Bisanz, J.M. [Eds], Preparing for a changing climate– The potential consequences of climate variability and change, Great Lakes overview. USGCRP
- 4. Hanrahan J.L., Kravtsov S.V., Roebber P.J. (2010) Connecting past and present climate variability to the water levels of Lakes Michigan and Huron. Geophysical Research Letters 37:L01701. DOI: 10.1029/2009gl041707.
- 5. Croley T.E.I. (2003) Great Lakes Climate Change Hydrological Impact Assessment: IJC Lake Ontario—St. Lawrence River Regulation Study. Technical Memorandum. Water Resources Management Decision Support. NOAA Great Lakes Environmental Research Laboratory, Ann Arbor, MI 126:84
- 6. Lofgren, B. M., T. S. Hunter, and J. Wilbarger, 2011: Effects of using air temperature as a proxy for potential evapotranspiration in climate change scenarios of Great Lakes basin hydrology. J. Great Lakes Res., 37, doi: 10.1016/j.jglr.2011.09.006