Section 1: Influences on Water Levels and Flows

1.1 Why do levels and flows fluctuate in the Lake Ontario-St. Lawrence River system?

The greatest influence on water levels is hydrology: the natural system of water storage, groundwater and stream-flow transport, precipitation, evaporation, formation and travel of clouds, and wind. Hydrology is directed and dominated by natural forces. The sole regulatory factor is the Moses-Saunders Power Dam, which, as you may discover below and in Section 2, is far less significant than the natural factors affecting water levels on Lake Ontario. Weather patterns, precipitation, and winds are major influences within the hydrologic system. The hydrologic system is worldwide, but the hydrologic effect on Lake Ontario is mainly the outflow of Lake Erie, the precipitation and evaporation over Lake Ontario, and the runoff from Lake Ontario’s local drainage basin. The vast majority of water supply into Lake Ontario is outflow from Lake Erie. This outflow is uncontrolled and is a function of the larger hydrologic system of the Great Lakes upstream.

The Lake Ontario system contains three major cycles:

1. Persistently high or low precipitation over several years is the main natural factor causing extreme high or low Lake Ontario levels. Prior to regulation, Lake Ontario experienced extreme low levels in the mid-1930s and extreme high levels in the early 1950s.

2. In spring, the melting snow and spring rains increase runoff into the lake. However, the lake is cooler than the air above at this time of year. As a result, less water evaporates during the spring than in the fall and early winter. With more water entering the lake than leaving, water levels usually rise, ultimately reaching their peak in the early summer. In general, water levels in Lake Ontario are lowest in the late fall and early winter. At this time of year, water on the surface of the lake is warmer than the air above. As a result, water evaporates rapidly. With more water leaving the lake —in the form of water vapor — than entering, water levels decline. This annual cycle may be seen in the plot of preproject water levels.

3. Within this seasonal variation, water levels may also change in a matter of hours because of wind effects.

1.2 Water from Lake Erie, precipitation, and wind are the major natural forces affecting Lake Ontario. How significant are these forces, and why?   

The three most significant natural forces affecting water levels in Lake Ontario are the inflow from Lake Erie, precipitation and evaporation, and wind.

 Inflow from Lake Erie: On average, Lake Erie supplies about 85% of the inflow into Lake Ontario. The majority of flow from Lake Erie to Lake Ontario is conveyed via the Niagara River. Inflows to Lake Ontario from the Niagara River, which are unregulated, are determined by Lake Erie water levels. Although Niagara River flows are fairly constant, with an 11% variation during the year on average, the inflow into Lake Ontario is typically highest in June and lowest in February, corresponding to the levels on Lake Erie.

 Local Precipitation and Evaporation: Precipitation (rain and snow) over the lake, the runoff from the precipitation over the local drainage basin, and evaporation of water from the lake surface account for, on average, the remaining 15% of the water supply. During late fall and early winter, evaporation from the lake surface can exceed inputs of water due to precipitation and local runoff, causing a net negative local water supply over a given period of time.

 Wind: Strong, sustained winds from one direction may push the water level up at one end of Lake Ontario, causing the level to go down by a corresponding amount at the opposite end. The effect is known as a “surge”. This is a short-term fluctuation, changing in a matter of hours. Once the sustained winds subside, the water will oscillate back and forth in the lake and bays until it levels itself out, much as it would in a bathtub. This is known as “seiche”. On Lake Ontario, wind surges have raised the local levels by as much as half a metre (1.5 feet). Wave “runup” can also raise local levels suddenly and significantly, worsening impacts that may already be occurring due to calm-weather flooding and erosion. In general, wind effects have a maximum duration of a few days and do not usually affect the weekly regulation of outflows by the Board.

The Board has no control over any of these three natural factors: the inflow from Lake Erie, the precipitation and evaporation, or the wind. Additionally, seasonal variation can influence the regulation of outflows from Lake Ontario, as naturally occurring weather conditions are beyond human control. For more details, the interested reader may wish to peruse the documents available on the webpage: Library.

1.3 What influence does isostatic rebound have on what is happening to water levels and water access around the Lake Ontario shoreline?  

Overall, isostatic rebound, which is the process whereby the earth’s crust is slowly adjusting to the lack of the weight of the glaciers from the last ice age, affects the north shore as well as the south shore of Lake Ontario. In general, the west end of the lake is sinking relative to the outlet, the St Lawrence River. Isostatic rebound means slightly deeper water for the northwest shore (15 cm) and for the southeast shore (4 cm) for the same given water level compared to 100 years ago.

1.4 What influence does climate change, increased evaporation and other factors have on what is happening to water levels and water access around the Lake Ontario shoreline?

Several long-term processes could affect coastal and boating risks, and unfortunately none of them seem to be for the better. These processes include isostatic rebound (a certainty); the possibility of more extreme water supply conditions (both wet and dry); the possibility of storms that are more severe (especially when there are higher atmospheric temperatures and water content); increased erosion impacts in winters when there is less ice along the shoreline; and increased erosion on unprotected parcels due to reductions in sediment transport resulting from shoreline protection on adjacent parcels. In addition to these long-term processes, short–term effects, such as wind setup, temporarily, but at times drastically, affect water levels, particularly on the south shore of Lake Ontario.

1.5 What are the overall actual and potential impacts of these changing conditions on the north shore of Lake Ontario?  

The north shore is somewhat less vulnerable to the effects of wind and waves due to prevailing weather patterns. The north shore is also somewhat less susceptible to flooding impacts because of more proactive coastal zone management practices.