Bill Andresen is chairman of the International Great Lakes Coalition, which claims 4,000 members in the United States and Canada. The group contends lake levels already are manipulated to serve commercial interests, and that governments could do more to prevent extreme variations. State, federal and international agencies that deal with the lakes respond that human tinkering is a drop in the bucket when it comes to determining water levels."The lakes are not bathtubs. The water levels have been changing for thousands of years. We have to accept that and adjust to it," says Roger Gauthier, supervising hydrologist with the U.S. Army Corps of Engineers in Detroit.