Section 3: Influences on Water Levels and Flows


3.1 Why do levels and flows fluctuate in the upper Great Lakes?

The greatest influence on water levels is hydrology: the natural system of water storage, groundwater and streamflow transport, precipitation, evaporation, formation and travel of clouds, and wind. Hydrology is directed and dominated by natural forces. The sole regulatory factor is the set of control structures located at Sault Ste. Marie, which, as described below and in Section 2, is far less significant than the natural factors affecting water levels. Weather patterns, precipitation, evaporation, and winds are major influences within the hydrologic system. The hydrologic system is worldwide, but the hydrologic effect on the upper Great Lakes is mainly the precipitation and evaporation over each lake and the runoff from each lake’s local drainage basin.

Lake levels tend to vary over three main temporal scales:

1. Long-term variations (years/decades):  Over-the long term, water levels fluctuate primarily as a result of variations in hydrologic conditions.  Precipitation patterns are the main natural factor affecting water levels:  periods of wet conditions result in increasing water levels, while dry periods result in water level declines.  Persistently high or low precipitation over several years is the main natural factor causing extended periods of relatively high or low lake levels, and combined with long-term trends in evaporation, over the long-term these natural factors may result in both high and low water level extremes.

2. Medium-term variations (months/weeks):  Water levels also vary seasonally, as hydrologic conditions change within the year.  In spring, the melting snow and spring rains increase runoff into the lake. The lakes are 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 lakes than leaving, water levels usually rise, ultimately reaching their peak in the summer. In general, lake levels tend to decline in the fall, reaching their lowest point in the late winter. During this time of year, water on the surface of the lake remains warmer than the air above.  As cold, dry air passes over a lake, it is warmed by the lake’s water surface and picks up water vapor as a result.  With more water leaving the lake - in the form of evaporation - than entering, water levels can continue to decline.

3. Short-term variations (days/hours):  Within this seasonal variation, water levels may also change in a matter of days or hours because of wind effects. See section 3.2 for more information.

3.2 Water gained from precipitation and lost from evaporation, and wind are the major natural forces affecting lake levels. How significant are these forces, and why?

The three most significant natural forces affecting water levels are precipitation, evaporation, and wind.

Precipitation: Precipitation (rain and snow), including that which falls directly onto the lake’s surface and that which falls onto the lake’s local drainage basin and enters the lake as runoff, together account for a significant amount of the water supply to each lake. Please note that even though rainfall or snowfall may seem particularly heavy or light in your immediate area, it’s the total amount over the entire lake system that determines the impact on water levels.

Evaporation: During late summer through late 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. A lake can easily lose several centimetres (inches) of water o2ver the course of one cold, dry night.

Wind: Strong, sustained winds from one direction may push the water level up at one end of a lake, 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 some lakes, wind surges have raised the local levels by as much as several metres. In general, wind effects have a maximum duration of a few days and do not usually affect the regulation of flows by the Board.

The Board has no control over any of these three natural factors. Additionally, seasonal variation can influence the regulation of flows from Lake Superior, as naturally occurring weather conditions are beyond human control. For more details, you may wish to peruse the documents available on the Reports page of the Board website

3.3 What is glacial isostatic adjustment and how does it influence lake levels and water access around the shorelines of the upper Great Lakes?

Glacial isostatic adjustment (GIA), also known as post-glacial rebound, 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. Due to variations in the thickness of the glaciers, the timing of the glaciers receding, the geology of the region and other differences, the rate that the earth’s crust is adjusting varies throughout the Great Lakes region, with some areas rising faster than others and some areas even falling relative to other locations.  This is reflected in the water levels of the Great Lakes.  In general, the south shore of Lake Superior is sinking relative to the lake’s outlet at the St. Marys River, while the north shore is rising relative to the outlet; similarly, the southwest areas of Lake Michigan are sinking relative to the outlet of Lake Michigan-Huron at the St. Clair River, while the northeast shores of Lake Huron are rising relative to the outlet. As a result, for the same-lake-wide average water level, over an extended period of decades or more, GIA means that, relative to the shoreline, water will appear deeper at certain locations, such as Duluth (+25 cm/century) and Chicago (+10 cm/century), while it will appear shallower at others, such as Rossport (-28 cm/century) and Parry Sound (-25 cm/century).

3.4 What influences do climate change and other long-term factors have on water levels and water access around the 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 set-up) temporarily, but at times drastically, affect water levels.

3.5 What is the significance of the Long Lac and Ogoki Diversions? What about the Chicago Diversion?

Lake Nipigon naturally drains into Lake Superior. The outflow from this lake has been regulated for hydropower production since the 1920s. Starting in 1943, water has been diverted from the Ogoki River (which drained through the Albany River into James Bay) into Lake Nipigon. This diversion, in conjunction with the nearby Long Lac Diversion, was developed to generate additional hydroelectric power. These diversions were authorized by an exchange of notes between the governments of Canada and the U.S. in 1940 and are not under the jurisdiction of the International Joint Commission. The Board obtains the records of these diversions from Ontario Power Generation for use in its regulation activities. Water is diverted into Lake Superior via these two diversions at an average rate totaling approximately 153 cubic metres per second (m3/s) or 5,400 cubic feet per second (cfs).

With low water in the early part of the 21st century, and the approach of Asian carp into the Great Lakes region, public interest in the Chicago Diversion has been high in recent years. The continental divide separating the drainage basin of Lake Michigan from that of the Mississippi watershed passes within 16 km or 10 miles of Lake Michigan west and southwest of Chicago. A canal linking the two systems was first completed in 1848. Several modifications were made, and diversion rates increased steadily to a maximum of 285 m3/s or 10,000 cfs of water removed from Lake Michigan in 1928. Other Great Lakes states and Canada objected, citing an impact on water levels. In 1967, Illinois agreed to limit the total diversion to 3,200 cfs (91 m3/s).

Therefore, more water is diverted into the upper Great Lakes than is diverted out.

Closing the O’Brien and Chicago locks (such as in response to the Asian carp threat) and sealing off the Chicago Diversion would result in little change in water levels. The IJC estimates that the total Diversion lowers Lake Michigan-Huron by 6 cm or 2.4 inches, and given that less than 15% of diverted flows pass through these locks, the lake level rise following permanent closure would be less than 1 cm or 0.4 inches.

3.6 How does ice affect lake evaporation and lake levels?

The Board’s understanding of the complex relationships between ice cover, evaporation and water levels is evolving. Evaporation causes water levels to decline. A commonly held belief is that high ice cover results in generally lower amounts of evaporation. 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. 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 levels.

As well, it takes a lot of energy to melt ice and snow, and ice and snow reflect solar radiation better than dark water does. During these same years with high ice concentrations, the water tends to stay colder than normal heading into the spring. This has 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.

3.7 Doesn’t knowing the snowpack provide a reliable indicator of the water supplies for the spring and summer season?

The correlation between the snow pack in a lake’s local drainage basin and its subsequent spring and summer level is very low. The reason is that there are many other factors that need to be considered and which are difficult to estimate or predict, including how frozen the ground is when the snow melts, how dry the soil is, how fast the snow melts and whether the snow sublimates (that is, evaporates directly from snow into water vapour without first melting into liquid). Most crucial is whether it rains at the same time as the snow is melting, which increases runoff dramatically.