International Lake Ontario - St. Lawrence River Board

Why are water levels low?


Following an unusually dry and cool February and March, water levels in the Lake Ontario-St. Lawrence River system were lower than the long-term average at the start of April. Until April, Lake Ontario had received relatively little in the way of precipitation on the lake or the surrounding lake basin this year. In fact, the net water supplies to the Lake Ontario basin (i.e., the net amount of water the lake receives in the form of precipitation and runoff, minus evaporation) were near record lows in the months of February and March. Lake levels are primarily determined by water supplies; so as a result, Lake Ontario is currently below its normal seasonal average. As in the past couple of years, this April has been somewhat wetter and water levels have risen sharply, yet still remain below average. The level on 29 April 2015 was 74.76 m (245.28 ft), which is about 21 cm (8.3 in) below long-term average for this time of year. This is well within Lake Ontario’s 1.22 m (4 ft) range, being 61 cm (24.0 in) below the lake’s upper limit, and 61 cm (24.0 in) above its lower limit which started in April. Lake St. Lawrence, just upstream of the Moses-Saunders powerhouse, was about 6 cm (2.4 in) above average.

The Ottawa River freshet influences St. Lawrence River levels downstream in the system near Montreal. The Ottawa River’s flow began to rise at the beginning of April, but has now peaked since most of the snow in the basin has already melted. As a result, on 29 April 2015 the levels at Lake St. Louis were 46 cm (18.1 in) below average, but 69 cm (27.2 in) above the Seaway Low Alert level, and at Montreal Harbour, levels were 83 cm (32.7 in) below average, but 1.19 m (46.9 in) above chart datum.

During the winter months, the International St. Lawrence River Board of Control (Board) did deviate from Plan 1958-D flows to assist in ice management and the transit of two ships. Since late July 2014, over-discharges from Lake Ontario, relative to outflows specified by the Lake Ontario Regulation Plan, have resulted in no more than a 0.8 cm (0.3 in) decrease in water level at any given time. Since Friday, 1 May 2015, water levels on Lake Ontario are at the levels prescribed under continuous Plan flows. The Board recently reviewed conditions in the system at their April Board meeting. The Board has decided to continue to follow Plan flows. The Board will also continue to permit additional under- or over-discharges that may be necessary to address unforeseen critical conditions in the river.

The Board, in conjunction with its staff, continues to monitor the system closely and is prepared to take action as required. Outflow changes are posted to the Board’s Facebook site at and its website at under the Maps & Data tab, Lake Ontario Outflow Changes.

Water levels on both Lake Ontario and the St. Lawrence River vary considerably from year to year depending on the weather conditions. The Board urges everyone to be prepared to live within the full range of levels that have occurred. Although the Board strives to maintain the range of monthly mean levels of Lake Ontario below the upper limit of 75.37 m (247.3 ft) and above the lower limit (from April through November) of 74.15 m (243.3 ft) specified in the Orders of Approval, since regulation began in 1960, actual monthly levels have ranged from a high of 75.73 m (248.5 ft.) to a low of 73.82 m (242.2 ft) due to climate conditions outside the design range. Levels on the river tend to vary more widely. Furthermore, excessive wind set up and wave action may significantly increase or decrease local levels on both the lake and river. Strong winds can change water levels temporarily by over half a metre (2 feet) in some locations.


Gail R. Faveri: (905) 336-6007;

Arun Heer: (513) 684-6202;


The International St. Lawrence River Board of Control was established by the International Joint Commission in its 1952 order of approval. The Board’s main duty is to ensure that outflows from Lake Ontario meet the requirements of the IJC order; it also develops regulation plans and conducts special studies requested by the IJC. For more information, visit To receive a weekly e-mail 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.