Lake Ontario lower this year despite outflow reductions to help prevent flooding

By on May 14, 2018

Water levels across the Lake Ontario – St. Lawrence River system have now begun their typical spring rise, but several months of high outflows combined with favorable winter weather conditions has levels back within their normal range and well below those seen during the historic high water event that began about a year ago. Outflows from Lake Ontario continue to be set according to Plan 2014 and, even with recent reductions necessary to ensure safe navigation and to reduce flood risks in the St. Lawrence River, they remain above average in response to above-average levels of both Lake Ontario and the upper Great Lakes.

As is typical in spring, outflows from the Ottawa River into the St. Lawrence River have risen significantly in recent days in response to increased snowmelt and springtime precipitation. However, the Ottawa River Regulation and Planning Board recently noted that, based on the current snow on the ground and the weather forecast, Ottawa River outflows are expected to remain within the normal range of fluctuations for this time of year. Accordingly, the International Lake Ontario-St. Lawrence River Board has temporarily reduced outflows from Lake Ontario to balance the risks and impacts of upstream flooding and erosion (on both Lake Ontario and in the Thousand Islands), with the risks of similar impacts downstream in the St. Lawrence River from Montreal through to Three Rivers. The flow adjustments are being done in accordance with Plan 2014 rules which reflect operational practices developed over the decades to balance upstream and downstream flood risks.

At this time, the risk of significant flooding either upstream or downstream remains low. Water levels across the basin are expected to continue their seasonal rise over the next several weeks, but there are currently no indications that the extreme rainfall that resulted in record Lake Ontario flooding and Ottawa River flows, and flooding along many areas of the lower St. Lawrence River in 2017, will occur again this year.  Flow reductions from Lake Ontario are expected to be temporary, and as the Ottawa River and levels downstream on the St. Lawrence begin to subside, outflows from Lake Ontario can be increased again.

The board notes that weather and hydrologic conditions play a more predominant role than water regulation in influencing water levels, and while impossible to predict, the probability of a repeat of last spring’s exceptional rains and subsequent high water levels is low. Nonetheless, extreme conditions may occur at any time in any given year. During extreme rainfall events, water levels can rise quickly throughout the Lake Ontario – St. Lawrence River system and may be amplified by wind forces. The board can do little to prevent this occurrence through outflow regulation, and so it urges communities to contact their local government officials and emergency managers and identify an emergency action plan to protect their property should these conditions occur. Likewise, low water impacts can also occur during periods of dry conditions and severe droughts, so shoreline property and business owners, recreational boaters and local government officials, should always be prepared for the full range of water levels on Lake Ontario and the St. Lawrence River in the future.

The board, in conjunction with its staff, continues to monitor conditions on an ongoing basis. Information on hydrologic conditions, water levels and outflows, including graphics and photos, are available on the board’s website and posted to the board’s Facebook page at

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