With 18 percent of the world's fresh water, the lakes slake the thirst of 45 million North Americans and sustain more commercial shipping than the Panama and Suez canals combined, but the low levels hampered ship traffic.

After several mild winters that increased winter evaporation rates and reduced the snowpack, water levels rose following heavy rains last fall and a once-in-a-century deluge in May. But that rainfall was an anomaly.

"That's what's worrisome," said hydrologist Cynthia Sellinger of the National Oceanic Atmospheric Administration.

"What made the lakes rise this year was we had an extremely wet fall and a wet May. What usually gives us a good rise in the lake is ice cover to prevent evaporation in winter, and then a nice snowpack melting in the spring thaw," she said.

During colder winters, ice covers the lakes and slows evaporation but this is now rare, said Joel Brammeier of the Lake Michigan Federation, an environmental group.

"In the long term, no one's quite sure what the implications for global climate change are for the Great Lakes," he said.

In the mid-1980s, when the water in Lake Michigan rose to near-record highs, storm-driven waves damaged shoreline high-rises in Chicago and undermined beach houses in Indiana and Michigan. Chicago is in the midst of a huge project to build concrete revetments to replace crumbling limestone blocks.

For aquatic life, rebounding water levels give fish access to habitat enriched by new plant growth from when water levels were lower, Brammeier said.