Canada holds a tenth of the world's fresh water, but also has 117 municipalities that reported water restriction days in just one year, according to our Environment Minister!
Great Lakes water levels have been low for some years, forcing light loads for Great Lake ships and groundings for pleasure craft. Marinas have difficulty providing enough depth for their customers and tourists. When water levels are very low, harbour dredging increases, even though dredging stirs up silt that can spread and smother fish spawning grounds. It can destroy fish habitat, killing them off even if the sediment is clean. If the sediment is polluted with metalic sludge or industrial waste, it can become a danger to people whose drinking water is taken from the waters.
The Great Lakes levels can be affected by a seiche (pronounced "saysh") caused by wind or air pressure changes that cause the water surface to rhythmically rock back and forth. It can be unnoticable or can cause boats to be left sitting on the bottom! Lake Erie is particularly affected by wind-caused seiches because of its shallow depth and long shape oriented to prevailing winds. These can lead to extreme seiches of up to 5 m (16') between opposite ends of the lake. In 1995, a sudden seiche on Lake Superior caused the water level to fall and then rise by 1 metre (3') within fifteen minutes. It should also be noted that large seiches also cause water currents strong enough to affect navigation. Rhythmic seiches have also been reported in the Napanee River, in the Quinte region of Lake Ontario.
The St. Lawrence River level can be affected by strong east winds, which can slow the eastward flow and lower water levels after many hours. (We woke one morning aground at a dock in the Thousand Islands!)
If the mean water level in Lake Ontario is less than .5 metres above datum, sailors in the Thousand Islands (sigh) get stuck in the mud, power-boaters get propellers repairs, and by autumn, marina operators can't get boats close enough to shore to lift out for winter!
Canal, such as the Rideau, Trent-Severn, and Chambly, are controlled by dams and locks, and normally do not vary except in spring flood season or after heavy rains. You should contact the canal before visiting if depth (maximum 1.5 m) is a concern!