Why Are the Great Lakes Low?

When there’s less water in Lake Michigan, the water level drops.

That part is easy – more water means the lake encroaches on the shore, and less water means bigger beaches, dredged harbors and docks towering feet over the water.

Right now we’re working with the “less water” scenario. In March, Lake Michigan was over two feet lower than average levels. In March 2012, the lake was 13 inches higher than it was this year.

The levels are low because there’s less water in the lake. The Great Lakes’ water supply is determined by this simple formula: precipitation + runoff – evaporation = net basin supply.

So when there’s little snow in the region to melt into the lakes, drought keeps spring and summer rains light, and hot temperatures keep the lakes warm and primed for lots of evaporation, the net basin supply drops.

“Starting around the late 1990s, the net supply of water going into Lake Superior was way below average,” said Andrew Gronewold, hydrologist for the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory (GLERL). “The same is true for Michigan and Huron. For most of those years the net supply of water has been way below average.”

In order to learn more about water levels and the net basin supply, GLERL built the Great Lakes Water Level Dashboard. The tool is online and interactive, and users can use it to see each lake’s historical water levels, precipitation, runoff and evaporation rates over time, and predictions for future levels.

“There’s some compelling evidence that abrupt changes in net basin supply [due to decreased precipitation and increased evaporation around 2000] led to a significant decrease in water levels during that time,” Gronewold said.

Jia Wang, GLERL ice climatologist, said increased evaporation is most to blame for our low lake levels. A warmer climate has led to warmer water temperatures and less ice on the lakes, which then led to more evaporation in the fall when cool air swoops in and sucks warm water off the lakes.

“This is caused by two factors,” Wang said. “One is the natural climate variability [or weather], and we can not do much about it because this is a year-to-year change… The other factor is climate change.”

Wang documented a steady rise in air temperature in the Great Lakes region – an indicator of a changing climate.

“We found that in the Great Lakes the surface air temperature increased at a rate of .4 to .6 degrees Celsius per decade over the last four decades,” Wang said. “That will turn out to be about 1.6 to 2.4 degrees warmer today than 40 years ago.” (That’s 2.9 to 4.3 degrees Fahrenheit.)

While climate and weather patterns drive the net basin supply formula, there are some other factors that affect water levels as well. Keep reading to learn more about them.

DREDGING

Water flows out of Lakes Michigan and Huron into Lake Erie through the St. Clair and Detroit Rivers, between Michigan and Canada. Those channels were dredged a few times between the late 1800s and 1960s, according to the Army Corps of Engineers, which caused a water level drop of about 20 inches.

But we haven’t seen those 20 inches since well before the 1960s.

“There are a whole number of factors that affect water levels, but the key factor affecting them right now is the climate,” said John Nevin, International Joint Commission Public Information Officer. “[Dredging] certainly has had an impact, but it’s a fixed impact. It isn’t something that changes over time.”

DIVERSION

Naturally, the only way water would leave the basin would be through evaporation and flowing out to the ocean through the St. Lawrence Seaway. In a number of places around the basin, man-made diversions have been made to send water into the Great Lakes or to take it out.

“When you balance all that out… it’s pretty much a wash,” said Jim Lubner, adjunct professor at the University of Wisconsin – Milwaukee School of Freshwater Sciences. “Diversion into and out of the basin is basically a balance and there are no additional diversions that I know of that are going to be allowed without the agreement of all eight [Great Lakes states] governors.”

GLACIAL ISOSTATIC ADJUSTMENT

Lake levels aren’t just determined by the atmosphere above – they’re also shaped from below. Scientists have observed a shifting of the Great Lakes basin’s geology as the earth rebounds from the weight of the glaciers that formed the region tens of thousands of years ago.

Different regions of the basin rebound at different speeds because of the varied weights of the glaciers. When one region of the basin rebounds faster, water could be distributed to areas with slower rebounding.

An Army Corps of Engineers map (see resources sidebar) shows the northern areas of the basin rebounding faster than the southern.

“The whole region is actually tilting,” Nevin said. “If you were to try to use regulation to solve the problem [and build a structure to slow the water flowing out of Lake Michigan] you might end up flooding Milwaukee.”

So now that we know the climate is largely in control, let’s move on to the second question: Can We Get the Water Back?