Why the Great Lakes are shrinking

The U.S. Army Corps of Engineers announced this week that two of America's Great Lakes, Lake Michigan and Lake Huron, are at their lowest water levels since recording began in 1918. The lakes were 29 inches below their long-term average, and down 17 inches since this time last year. "We're in an extreme situation," the Corps' Keith Kompoltowicz told USA Today.

The causes are threefold:

* Below-normal levels of rain and snowfall

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* Record-high temperatures

* Dredging (the process of deepening navigational channels by removing layers of the bottom sediments)

Why should we be concerned? First of all, the Great Lakes store 84 percent of the nation's freshwater, so their decline should be cause for some alarm. Second, "a long-term decline in water levels threatens coastal habitats, especially wetlands," says Robert Glennon at the Detroit Free Press. Plus, lower water levels are bad for the local economy. For every inch the water level falls, cargo ships have to lighten their loads an average of 100 tons to avoid running aground. This dramatically boosts the costs of shipping.

Researchers are eager to find a way to prevent water from leaving the lakes. One option: Tightening regulations on groundwater pumping, which sucks up runoff that feeds the lakes. "The state can't control precipitation, but it can control diversions from streams and pumping from wells that reduce flow into the Great Lakes," says Glennon. The Army Corps has suggested placing "speed bumps" in the surrounding rivers to slow down lake drainage, which could be put in place over the next few years. But what the lakes really need is several consecutive years of above-average rain and snowfall — and there's no magic wand for that.