The water levels of the Great Lakes fluctuate widely from season to season and from year to year. The summary of lake level data for Georgian Bay, presented in this graph, gives a clear picture of the nature of the fluctuations that have been observed over the years.

Perhaps as we have returned to our cottages each year we have grown accustomed to the water level in the Bay being up or down 25 to 50 centimetres from the last year (i.e. 1 to 2 feet). Certainly, as indicated in the graph, yearly fluctuations of such amounts have been quite common, and most of the time a drop in level for a year or two has been followed by a rise in the next couple of years or vice versa.

However, the graph also reveals that the water level of the Bay has fluctuated over a range of almost 2 metres during the past 140 years, including levels as low as or lower than the present ones on several occasions. Water levels were relatively high in the late 1800s, late 1920s, early 1950s, mid 1980s and mid 1990s; and they were particularly low in the mid 1920s, the 1930s, the late 1950s, the mid 1960s and now.

The present low levels may have caught our particular attention because the drop in level over the last 5 years has seemed continuous and the rate of drop quite dramatic. Water levels have fallen almost 1.5 metres in about 3 years! However, such rates of change in level, both up and down, have occurred before on the Bay - not often, but nonetheless they have occurred. For example, the extremely rapid rise in water level in the late 1920s and subsequent drop in the early '30s - up and down 1.5 metres in 8 years - is definitely noteworthy. And the rise of 1.3 metres in the early Ô50s followed by a drop of 1.5 metres in the mid to late '60s - another significant up and down in about 10 years - must also have seemed pretty dramatic, particularly when these fluctuations were followed by a rise of a metre and another drop of more than a metre in the subsequent 5 years.

So yes, the water level in the Bay has fluctuated over the years - and at times very significantly and rather rapidly. How is this possible? What causes and controls such fluctuations anyway?

Day by day - even hour by hour - water level fluctuations, even of several centimetres, can be caused by wind and barometric pressure differences across or down the Bay. Month to month and seasonal changes in level are due to the precipitation coming as rain in the warmer months with the associated runoff, and as snow in the winter months with delayed snowmelt. Also, evaporation from the Bay surface varies significantly throughout the year as the air and surface water temperatures change.

It is instructive to look at the so-called water balance for the Lake Michigan-Huron body of water which includes Georgian Bay. The dominant inputs to this lake system are: (i) precipitation falling on the lake surface, (ii) runoff from lands adjoining the lake, and (iii) the inflow into Lake Huron from Lake Superior. The average annual value (expressed as a depth of water over the entire lake surface) and the relative percentage contribution of each input are estimated to be: 815 mm of precipitation (40% of the input), 665 mm of runoff (32%) and 570 mm of inflow from Lake Superior (28%). These combined inputs contribute slightly more than 2 metres of water (2.05 m) per year on average to the Lake Michigan-Huron system, a volume that can be visualized as 2.05 metres of water over the entire water surface of Lake Michigan, Lake Huron and Georgian Bay, an area amounting to some 117,400 square kilometres. It can also be expressed as 240,670 million cubic metres of water, or a cube 20 metres on a side for every man, woman and child in Canada.

The dominant lake outputs include: (i) outflow to Lake Erie, and (ii) evaporation from the lake surface. The average annual values here amount to 1,390 mm (68% of the total outflow) of outflow to Lake Erie and 635 mm (31%) of evaporation. A relatively small amount of water, 25 mm (1%), is diverted annually through Chicago to the Mississippi River Basin for the dilution of sewage in that area.

How does all this relate to yearly fluctuations on Georgian Bay? Consider the possible conditions associated with a dry, warm year when the water levels in both the Lake Superior and Lake Michigan-Huron Basins are relatively low, as in recent years. The precipitation input to the Michigan-Huron system for such a scenario might be 610 mm (75% of normal), the runoff from adjoining land 465 mm (70% of normal), and inflow from Lake Superior 485 mm (85% of normal as it was in the year 2000). The resulting sum of inputs would be 1,560 mm, rather than the average annual input total of 2,050 mm. The outflow to Lake Erie for this scenario might be about 1,180 mm (85% of normal as in the year 2000), and lake evaporation 760 mm. Then the outputs from the lake system, including the 25 mm of diversion, would total 1,965 mm, a value exceeding the inputs by 405 mm. As a result, the water levels in Lake Michigan-Huron, including Georgian Bay, would drop by about 40 centimetres.

In fact, water levels in the Bay have dropped by at least 40 centimetres during some recent years. So the annual water balance can provide a very useful tool for gaining some appreciation for the order of magnitude of changes required to produce fluctuations in water levels akin to those observed in recent years. They are consistent with changes observed in the related weather variables.

To what extent can humans control the lake levels? Only two terms in the annual water balance can be directly affected by humans: the outflow from Lake Superior and the diversion at Chicago. There is no control mechanism at the outflow to Lake Erie. When water levels in Lake Superior are above average and levels in the Lake Michigan-Huron system are below average, more water than usual is released from the Superior basin, amounting to a difference in that input of a few percent. And when water levels in Lake Superior are below average and levels in the Lake Michigan-Huron Basin are high, then outflows from Superior may be reduced. However, when water levels in both lake systems are well below average, outflows are down anyway (as shown in the example above), and human control is almost irrelevant. Since the Chicago diversion amounts to only about 1 to 2 % of the total output from the system, changes in that term are also quite insignificant. Therefore, water level fluctuations on Georgian Bay, and particularly those of recent years, are attributable almost totally to changes in patterns of annual precipitation and associated runoff.

Is the Bay in danger of drying up? Not for quite a while! Although a 1 metre drop in the lake level over the Lake Michigan-Huron system involves the loss of a very large amount of water, this volume constitutes only about 1.4% of the volume of water stored in that lake system. It is indeed a vast water system.

So water levels in Georgian Bay, and in other areas of the Great Lakes, have dropped significantly in recent years, and have dropped at a reasonably rapid rate. They have done so before, and they are likely to do so again. They have also risen more or less dramatically on many occasions, and are likely to repeat that phenomenon too. The timing and magnitude of these fluctuations are determined almost entirely by variations in weather, and are destined to remain so. As our ability to predict the weather and the resulting fluctuations over the next few years is quite inexact, we will have to be content to observe, marvel or curse at them, as suits our lot.

(Comments and questions regarding this article are welcomed by the author at wdickins@uoguelph.ca)

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