The rise and fall of the Great Lakes.

The rise and fall of the Great Lakes 

For the last two years, the water levelsof Lakes Michigan, Huron and Erie have been at record heights. With the resulting disappearance of protective beaches, low-lying areas now bear the brunt of any severe storms that sweep across the lakes. The combination of high water levels and storm-generated wave action has already caused considerable flooding, erosion and damage to homes, docks and other shoreline structures.

The problem isn't likely to go awaysoon. "Even if we had a drought,' says Frank H. Quinn of the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory in Ann Arbor, Mich., "it would take Huron and Michigan three and a half years to return to the usual levels, and Lake Erie would take four years.' With normal precipitation and runoff, he says, the Great Lakes system would take six to 10 years to recede to more usual levels. Several wet periods, on the other hand, could raise water levels by another foot over the next few years. Quinn made these projections last week in Chicago at an American Association for the Advancement of Science meeting.

Although water levels have been risinggradually for the last two decades, the current record heights were caused by two consecutive years of abnormally high precipitation. It was one of the few times since the 1880s that all of the lakes had periods of high rainfall at the same time. In contrast, just 22 years ago, the Great Lakes' water levels were at record lows, about five to six feet below the present high levels.

Large water-level fluctuations over periodsof decades have often occurred in the past, says Quinn, and they're likely to continue. "This poses a real challenge for people living along the lakes and managing the lakes,' he says. "How do you cope with this type of a range in lake levels?' Moreover, he adds, "to date, we've been able to find no well-defined cycles which can be used to predict lake levels on into the future.'

The Great Lakes hold about 20 percentof the world's fresh surface water. Because of their large surface areas and restricted outlets, the lakes respond slowly to changes in precipitation, runoff and evaporation rates and to human efforts to control or divert flows. For instance, doubling the amount of water that is diverted from Lake Michigan into the Mississippi River system, says Quinn, would lower the mean water level at Chicago by merely an inch or so after about three years.

"This is one of the main reasons whydiversions have not been used to regulate the water levels,' he says. "By the time you make a change and . . . get the effect, times have probably changed, and the conditions that you originally set out to achieve are no longer valid.' Furthermore, such projects are costly, and diverting excess water into the Mississippi or slowing the flow out of Lake Superior may just shift where flooding occurs.

The historical record also shows thatthe period between 1930 and 1960, a time of considerable shoreline development, was perhaps the warmest 30-year period in the last 2,000 years or so. It also happened to be a drier-than-usual period.

"If we look at the last several thousandyears,' says Quinn, "the climate that we have today, which is cool and wet, may very well be the normal climate for the Great Lakes region. And the period that many of us considered to be normal, which included some very warm and dry periods, may very well be the abnormal climatic base.' In the same way, the geological record indicates that today's high water levels may be closer to the long-term "normal.'

Meanwhile, the lack of ice on the GreatLakes this winter means that storms could drive lake water against shorelines and cause severe flooding in lakefront cities. "The potential for problems,' says Quinn, "is much greater than it ever has been during the years we've been keeping water-level records.'