Impacts of the Extreme 2019 Great Lakes High Water Levels Felt Throughout Lake Ontario and the St. Lawrence River

Anthony M. "Tony" David
International Lake Ontario-St. Lawrence River Board
October 15, 2019
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When water supplies exceed capacity in the Lake Ontario-St. Lawrence River system, water levels rise. When this happens, regulation is looked to as the solution. But during periods of extreme water supply, the ability of regulated outflows to influence Lake Ontario levels is greatly diminished. In practical terms, no regulation plan can eliminate high water events on Lake Ontario or the St. Lawrence River.  

What regulation can do, when compared to conditions before the construction and operation of the Moses-Saunders Dam, is reduce peak elevation and shorten the recovery time following a high-water event. High-water events in the Lake Ontario-St. Lawrence River system have been observed repeatedly since the 1960s, which highlights the limitations of regulation in addressing extreme conditions. So rather than asking what regulation can do to prevent extreme flooding, the question should be: How can a more resilient shoreline be developed to lessen the impact of high-water events?

Water levels across the Great Lakes are primarily the result of natural, uncontrolled water supplies into the basin. Lake Ontario experienced a record-high water level in 2017 of 75.88 meters or 248.95 feet. It then exceeded that just two years later in 2019, with a new record of 75.92 meters or 249.09 feet.

Significant damages and other impacts were experienced across the system. Shoreline communities are seeking reliable ways to reduce property damages, protect critical infrastructure and maintain essential services. Coastal communities across Canada and the United States are taking adaptive action to improve resilience to extreme weather. Building resilience under this framework offers many coastal communities reliable outcomes and a means for improving development when performance falls short of expectations.   

The root cause of most flooding impacts is vulnerable development near what’s called the base flood elevation. The term is used by jurisdictions to examine regional flood elevations based on local conditions. So given the constraints of any regulatory plan, the most-reliable means to address flooding is through assessment of vulnerability and resilient design to an elevation near the applicable base flood elevation. 

Under extreme water supplies, regulation Plan 2014 is nearly identical to the previous Plan 1958-DD. The best outflow strategies used under the previous plan have been incorporated into the set of rules that govern outflows under Plan 2014. A report produced by the board following the 2017 high-water event compared water levels under Plan 2014 to former Plan 1958-DD and concluded the outcomes were nearly identical. A similar comparison will be prepared for 2019.

The levels observed in 2019 are the result of several separate extreme weather conditions occurring in the same year:

·         Persistently high flows from Lake Erie into Lake Ontario, eventually exceeding record-highs by spring 2019

·         Above-average precipitation within the Lake Ontario-St. Lawrence basin from late-fall through spring

·         A record-setting flood event on the Ottawa River that joins the St. Lawrence River near Montreal, the second in three years.

These conditions were the primary drivers that led to the record water levels on Lake Ontario and the St. Lawrence River.  

The board has made every effort to provide relief from the extreme water event, by continuously adjusting outflows during the several months of historically high water supplies leading up to the spring of 2019, and in consideration of the impacts on various interests throughout the system.

Since then, the board has increased outflows to record-high values in an attempt to lower the extraordinary levels of Lake Ontario and provide relief to those impacted, while also considering the impacts to shoreline property owners downstream on the St. Lawrence, and to other stakeholders, including commercial navigation and the industries it supports. This highlights the need for coastal communities and channel users to systematically prepare for extreme weather, water levels and flows (both for extreme highs and the eventual return of extreme low levels).

Further information on Lake Ontario flow regulation can be found on the board’s Facebook page.

watershed basin map lake ontario
Watershed basin map with outlet locations. Credit: Environment and Climate Change Canada


Anthony M. "Tony" David
International Lake Ontario-St. Lawrence River Board

Tony David is director of the Environment Division of the Saint Regis Mohawk Tribe and member of the Lake Ontario- St. Lawrence River Board.