By: P.F. Karrow
Figure 1. Short-term monthly water levels. This diagram is provided by the Canadian Hydrographic Service,Department of Fisheries and Oceans.
Anyone who has had a cottage or residence on the shoreline of the Great Lakes for a few years will be aware that water levels vary. Those involved with water transportation on the Great Lakes are very concerned about the changing levels because low water reduces the available draft for cargo in ships. Municipalities and individuals have to deal with shore erosion and property encroachment in high waters and boaters complain when their docks are submerged. They also complain when the water is too shallow to reach the dock in their boats.
Why do lake levels vary? Many factors affect lake level, some short term and some long term. Some factors are only local, while others affect the whole region. For hours or days, wind may lower water level on the upwind side of a lake and raise it on the downwind side. When the wind changes the water may slosh back and forth across the lake (called seiches) with changes of up to a few metres possible. Like the oceans and the crust of the Earth itself, there are daily tides, but these are normally too small to notice and only detectable with precise instruments.
Seasonally and over years and decades, the principal factor in water level changes is the weather - particularly the amount of precipitation. Also, temperature and humidity affect evaporation and the melting of winter snow and ice. Records show an annual cycle of level changes (a range of about 0.5m) with a peak in July and a low in February (Fig. 1). In 1997 levels were more than 0.5m above average, but because of drought in 1998-99, have been below average for the past year (about 0.3m in the summer of 1999). Because of the size of the Great Lakes region, the individual lakes do not vary consistently because weather factors may differ between lakes. Nevertheless, because they are linked by rivers, variations do tend to carry downstream through the system.
Figure 2. Long-term lake levels from 1918 to present. To Current records for this and other Great Lakes the web resource provided by Environment Canada.
We hear a lot about global climate changes ('warming'), thought by many to be caused by human activity. Records show longer cycles of change of about 1.5m (records exist for only about the last 150 years, Figure 2), again reflecting weather changes. For example, there is an 11-year sunspot cycle that affects precipitation. The record high levels (and destruction) of 1986 were followed by the next high in 1997, right on the sunspot schedule. Some projections for the future based on expected warming ind icate dropping water levels from increased evaporation.
Those same records for the past 150 years also show the Earth's crust is tilting, for example raising Kingston relative to Hamilton, Ontario, at a rate of 0.3m per century. Since the outlet of Lake Ontario (i.e. the controlling sill for water level) is n ear Kingston, that means that lake level at Hamilton is rising about 0.3m per century. In fact, records from geological history show that lake level at Hamilton has risen about 100m over the last 11,000 years! But that is another topic for separate discussion.