The month of May has been drier than average in the Great Lakes-St. Lawrence River watershed. Based on data from the GLERL Hydromet database and US Army Corps of Engineers data, the most recent 12 months over Lake Ontario have been the driest since 1966. The Canadian and United States drought monitor maps show abnormally dry to moderate drought conditions within the basin: https://www.ncdc.noaa.gov/temp-and-precip/drought/nadm/maps. As a result, the water level in Lake Ontario only rose 2 cm (0.8 in) in the month of May as compared with the average rise of 9 cm (3.6 in) in May.
“The level of Lake Ontario has declined below the low Criterion H14 threshold specified in the IJC regulation plan”, said Stephen Durrett, United States Co-Chair of the International Lake Ontario – St. Lawrence River Board. In accordance with the International Joint Commission (IJC) Directive (https://ijc.org/en/loslrb/who/directives/deviations), the regulation plan needs to be followed until water levels reach any of the defined triggers. Due to water levels decreasing below the low threshold that applies at this time of year, the International Lake Ontario – St. Lawrence River Board has implemented deviations from plan-prescribed “Rule Curve” flows.
On May 29th, outflows from Lake Ontario were set to 100 m3/s (3,530 cfs) below the amount prescribed by the “Rule Curve” of the regulation plan. Outflows will be further decreased on June 5 to 7,620 m3/s (269,100 cfs), which is 200 m3/s (7,060 cfs) below the amount prescribed by the “Rule Curve” of the regulation plan. The Board will continue to monitor conditions and the effects of the deviation strategy closely while simultaneously tracking weather forecasts and drought conditions.
Deviations from plan-prescribed “Rule Curve” flows under the low Criterion H14 threshold are meant to “provide all possible relief to municipal water intakes, navigation and power purposes, upstream and downstream.” Impacts to all interests were considered by the Board while exploring possible deviation strategies under this authority. The deviations are expected to decrease the risk of excessive low water levels caused by dry summer conditions. However, outflow management cannot eliminate the risk of extreme water levels from occurring during periods of severe weather and water supply conditions.
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 https://www.facebook.com/InternationalLakeOntarioStLawrenceRiverBoard (English), and more detailed information is available on its website at https://www.ijc.org/en/loslrb.