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The dynamics of regulation for the Lake Ontario-St. Lawrence River system are complex, requiring the balancing of several conflicting water management objectives that are inherent in the management of flows and lake levels. For example, alleviating high water levels on Lake Ontario requires releasing more water, which may cause flood-related damage downstream because of high water conditions in the River. Alleviating low water levels in the lower river requires releasing more water from Lake Ontario, which may cause problems for recreational boaters and municipal water supplies because of low water levels on the Lake. Managing the variability of water levels to accommodate ecosystem needs introduces a higher level of complexity.
Uncertainty about future water supplies from the Upper Great Lakes and tributaries within the Lake Ontario-St. Lawrence River Basin makes it that much harder to balance upstream and downstream needs. For example, if future supplies are unexpectedly low, releases made to alleviate low water levels in the River may drain too much water from Lake Ontario, making it much more difficult to alleviate low river levels later when the impacts may be even worse. Similarly, if future supplies are unexpectedly high, restraint in making releases to avoid minor downstream flooding may induce greater damage when later releases have to be increased dramatically.
The timing of water availability within the year is important, in different ways for different purposes. The level of commercial navigation and recreational boating activity drops considerably in the winter. The value of energy generated in the summer during peak energy demand periods can be more than twelve times the value in the spring. Higher releases reduce the level of Lake St. Lawrence, which is immediately upstream of the hydropower dam. If the releases are too high, the levels can be so low that they are hazardous to navigation and could result in ship groundings. In addition, high flows can produce cross-currents that make it difficult to control vessels. For hydropower, more electricity can be generated when there is a greater volume of water passing through the turbines; however, the consequent lowering of Lake St. Lawrence decreases the head on the hydropower stations and reduces the amount of electricity generated for each cubic meter of water.
The Orders of Approval require that riparian interests downstream receive no less protection from flooding than would have occurred under pre-project conditions. Regulation of Lake Ontario outflows has actually reduced spring flooding in the Montreal area, while still reducing flooding on Lake Ontario. Montreal is threatened by flooding since it is located at the confluence of the Ottawa and St. Lawrence rivers. The spring runoff from the Ottawa River Basin is largely uncontrolled and can be very significant. Timely adjustment of the Lake Ontario outflow has repeatedly helped avoid serious flooding around Lake St. Louis in the Montreal area during Ottawa River floods. Lake Ontario outflow reductions are typically offset by higher flows prior to the Ottawa River flood, or shortly following it.
