International Lake Ontario - St. Lawrence River Board

Section 1: Influences on Water Levels and Flows

  

 1.1 Why do levels and flows fluctuate in the Lake Ontario-St. Lawrence River system?
 1.2 Water from Lake Erie, precipitation, and wind are the major natural forces affecting Lake Ontario. How significant are these forces, and why?
 1.3 What influence does isostatic rebound have on what is happening to water levels and water access around the Lake Ontario shoreline?
 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?
 1.5 What are the overall actual and potential impacts of these changing conditions on the north shore of Lake Ontario?
 1.6 How does ice affect lake evaporation and Lake Ontario water levels?

 

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1.1 Why do levels and flows fluctuate in the Lake Ontario-St. Lawrence River system? [top of page]

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 Dam which, as you may discover below and in Section 2, is far less significant than the natural factors affecting water levels. 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, as shown in the plot of monthly lake levels shown below.

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 pre-project 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? [top of page]

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 80% 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 20% of the water supply. During late summer and fall, 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). In general, wind effects have a maximum duration of a few days and do not usually affect the weekly regulation of flows by the Control 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 flows 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 our bookshelf, available on the webpage: Publications.

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

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? [top of page]

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 set-up, 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? [top of page]

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.

1.6 How does ice affect lake evaporation and Lake Ontario water levels? [top of page]

Our understanding of the complex relationships between ice cover, evaporation and water levels is evolving.  Evaporation causes water levels to decline, that much we know.  A commonly held belief is that high ice cover results in generally lower amounts of evaporation. This is true to some degree, in that when ice covers much of the water surface, it acts as a cap on the lake, effectively preventing evaporation from occurring. But for that high ice cover to have formed in the first place, the water needed to cool (lose energy), and the most effective way to do that is through evaporation.

Lake evaporation is at its peak in the fall and early winter, when cold, dry air passes over the lake's relatively warmer water. So during years with high ice concentrations and reduced evaporation in late-winter, evaporation rates earlier in the season may have been higher, resulting in a rapid lowering of Lake Ontario water levels.  

As well, it takes a lot of energy to melt ice and snow, and ice and snow also reflect solar radiation better than dark water does.  So during these same years, the water tends to stay colder than normal heading into the spring, and this will have implications later in the year, possibly delaying the onset of evaporation and the typical seasonal decline in water levels until later in the summer and fall than they would normally tend to occur.