A new report by the Lake Ontario-St. Lawrence River Board provides a detailed account of the record-breaking flood of 2017, and what the board did to reduce the levels on Lake Ontario and the St. Lawrence River.
A key finding of the report is that outflow rates during 2017 were determined by extreme weather and record-setting water supply conditions.
During the winter of 2017, flows were set to avoid ice jams under highly variable temperatures. In spring and early summer, the flows were repeatedly adjusted to reduce and balance upstream and downstream flooding.
The board made every effort to minimize impacts and maintain the highest possible outflows without threatening navigation safety.
The report concludes that the board would have faced these same conditions under the previous regulation plan, and that outflows would have been very similar to those prescribed under Plan 2014, a new plan for regulating Lake Ontario outflows that went into effect in January 2017.
Lake Ontario started 2017 slightly below its long-term average level and rose a record-breaking 1.4 meters (about 4.5 feet) by late May.
This was due to extreme wet conditions in the Lake Ontario basin, including record precipitation in the April-May timeframe, and above-average inflows from the upper Great Lakes.
The widespread wet spring weather also led to record flows in May from the Ottawa River into the St. Lawrence River near Montreal and severe flooding conditions that extended further downstream.
Throughout the spring, the board was faced with releasing water from a flooding Lake Ontario into a flooded St. Lawrence River.
The record rise on Lake Ontario was followed by a record decline of 1.1 meters (about 3.6 feet) from the start of June through December.
This was due in part to record outflows from Lake Ontario set by the board during the summer and continued high outflows prescribed by Plan 2014 thereafter.
Declining inflows, including a much-needed dry spell at the end of August through September, were also major contributing factors.
Plan 2014 released significantly more water from Lake Ontario than would have been possible prior to the removal of bed rock to enlarge the St. Lawrence River channel when hydropower and Seaway projects were built in the 1950s. In 2017, the peak level on Lake Ontario would have been about 18 centimeters (7 inches) higher without regulation, and extreme high Lake Ontario levels would still be occurring as of June 2018. Without regulation of outflows, ice jams (that occurred more frequently prior to regulation) would likely have made the flooding worse. And peak levels downstream also would have been significantly higher without regulation.
The report relied on data provided by water management and weather agencies in Canada and the United States, including the National Oceanic and Atmospheric Administration, National Centers for Environmental Information, US Army Corps of Engineers, Environment and Climate Change Canada and Canadian Hydrographic Service.
The events of 2017 once again demonstrate the vulnerability of shoreline communities to flooding and erosion. To reduce the risk of future damages, the most effective approach is to make property and infrastructure more resilient to coastal hazards and redouble efforts to prepare for future high water events. This will be a long-term undertaking, but all levels of government and organizations such as the IJC with knowledge about coastal impacts must work together if we wish to reduce the risk of catastrophic damages from the next extreme event.
The board’s report, “Observed Conditions and Regulated Outflows in 2017” is available online, along with a short video on the Causes of the 2017 Lake Ontario-St. Lawrence River Flood.