MODELING WETLAND RESPONSE TO PROPOSED CHANGES IN HYDROLOGIC REGULATION OF A LARGE RESERVOIR – LAKE ONTARIO

Water levels of Lake Ontario have been regulated since the St. Lawrence Seaway began operation nearly 45 years ago, resulting in conversion of diverse wetland plant communities to cattail-dominated marsh and associated changes in faunal habitat. As part of an International Joint Commission study to develop a new regulation plan, data were generated from field studies to construct a GIS-based predictive model that assesses wetland responses to any new water-level regulation plans under consideration. Plant communities indicative of past water-depth histories were characterized in eight wetlands of each of four geomorphic types by sampling along transects that followed elevation contours with differing numbers of years since last flooding or last dewatering. Basin morphometries of the four geomorphic types were modeled in GIS using topographic/bathymetric data. Algorithms were developed to analyze proposed 100-yr hydrographs year by year, assign portions of the elevation gradient to plant communities with water-level preferences consistent with past flooding or dewatering as shown in the hydrographs, and determine the predicted area of wetland in each plant community type by reference to the geomorphic models. The 100-yr sequences of vegetation change were then compared to pre-regulation plant communities determined from photointerpretation of historic air photos to assess how well they mimic natural conditions. In collaboration with other vested interest groups, such as shoreline property owners fearful of erosion during storm events in high-water years and small boat operators fearful of low waters, a compromise regulation plan was developed for consideration with lake levels that do not exceed the post-regulation extremes. The plan provides wetland benefits by adding more lower lake-level periods and adjusting the frequencies between sequential high lake levels and intermittent lows to match natural net basin supplies more closely.