All water levels are referenced to International Great Lakes (Vertical) Datum 1985 (IGLD85).  Note that local communities and government agencies may use other datums such as NAVD88, CGVD28 or CGG2013 – particularly when determining flood risk.  Forecast water levels can also be subject to a high degree of uncertainty and, importantly, do not account for local variations due to wind and wave effects.  For info on local conditions, please refer to the responsible authorities in your area. 


Lake Ontario Water Level Forecast


This weekly forecast of Lake Ontario lake-wide average water levels and outflows is produced by the Great Lakes – St. Lawrence Regulation Office of Environment and Climate Change Canada as part of its responsibilities to provide engineering and technical support to the International Lake Ontario – St. Lawrence River Board. 

In simple terms, this forecast represents the projected range of water levels and flows that may be expected to occur under potentially wet, average and dry conditions.  Actual water levels and flows will depend primarily on weather and water supplies, and during periods of extreme conditions, may fall outside of the projected range shown here.  

This forecast employs an ensemble forecasting methodology, whereby a set of over 100 water supply scenarios are run through regulation and routing models to produce a corresponding set of simulated water levels and flows.  Historically observed Lake Ontario net basin supplies are adjusted based on guidance from short-term (1 to 4 week) meteorological forecast model output. These adjusted net basin supplies and corresponding ensembles of forecasted inflows from Lake Erie, inflows from tributaries to the St. Lawrence River (including the Ottawa River), as well as other inferred and forecasted inputs are used to simulate over 100 possible water level and outflow scenarios.  The ensemble of model results is post-processed and summarized to provide this probabilistic forecast of water levels and flows expected to be exceeded 5%, 50% and 95% of the time. 

Note that monthly forecasts of projected Great Lakes levels are also available and jointly produced by Environment and Climate Change Canada and the U.S. Army Corps of Engineers, Detroit District, under the auspices of the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data.  These bi-nationally coordinated six-month forecasts are derived using similar tools and methodologies to those described here, and are published in monthly bulletins through the responsible agencies in both Canada and the United States: