Moses-Saunders Dam

Background Paper Answers Key Questions on 2019 Lake Ontario and St. Lawrence River Flooding


In an easy-to-read background paper on High Water in 2019, the International Lake Ontario-St. Lawrence River Board responds to the call for more accessible information.

The paper uses question and answer format to explain why it flooded in 2019, why Lake Ontario outflows were not higher in 2018 and 2019, how the Board’s actions affected water levels, and what actions are being taken to reduce the risk of flooding in 2020 and beyond.

The factors causing the extraordinary spring rise on Lake Ontario in 2019 were similar to those in 2017, but there were differences. Record-high inflows from Lake Erie contributed more water in spring 2019, while record-setting rainfall on the Lake Ontario basin was a larger factor in spring 2017. More information is also available in the 2017 High Water Questions and Answers paper.

The total amount of water released from Lake Outflows from January 2017 through December 2019 was the highest on record for any 36-month period, however, total inflows were also the highest on record. Outflows from Lake Ontario have consistently been kept very high since the flooding of 2017, with the exception of temporary reductions during the springs of 2017 and 2019 when flooding was occurring both upstream and downstream. 

Regulation of Lake Ontario outflows reduces the severity of high water impacts on both Lake Ontario and the lower St. Lawrence River.  In 2017 and 2019, it reduced the peak and duration of flooding; in 2018, it helped prevent flooding altogether.

Outflows will remain high in 2020.  The Board has authority to continue deviating from the current regulation plan, Plan 2014, even after Lake Ontario falls below levels that would normally trigger such authority. This will allow the Board to further increase outflows whenever opportunities arise considering the impacts that these flow increases will have on other interests of the system.



Rob Caldwell:              (613) 938-5864                          

Susie Blair:                  (716) 879-4410                          


For more information, please see the Board’s website ( and Facebook page ( receive a weekly email about water levels and flows in the Lake Ontario–St. Lawrence River system, please send a blank e-mail message to with the word ’subscribe’ in the title and body of your message.