Top ten weather stories for 2007: story seven
7. Great Lakes - How Low Will They Go?
Trillions of litres of water have recently vanished from the Great Lakes -- a system containing 20 per cent of the world's fresh surface water. In September, Lake Superior set a record for its lowest water level for this time of year since measurements began in 1900. Persistent drought and warmer temperatures pushed levels 10 cm beneath the previous monthly low reached in 1926. Further, water flow from its outlet on the St. Marys River plunged, which bodes ill for the Great Lakes system because Lake Superior is the single largest source of water replenishing the four downstream lakes and the St. Lawrence River.
By December 1, water levels on Lakes Michigan-Huron were also sagging, more than two-thirds of a metre below average and about a metre lower than a decade ago. Water levels were just 8 cm higher than their 1964 record low for the same time and the lowest they have been in December since then. Whereas Ontario and Erie were down one-quarter and one-fifth of a metre from the long-term average, respectively, levels of the St. Lawrence River at Montreal were more than one metre below average -- its lowest since modern records began in 1967.
But it was Lake Superior that garnered most of the attention and explanation. Its levels have been consistently below average for 10 years. More telling though, from June 2006 to May 2007, water supply to the lake was the lowest for any 12-month period on record. Of note, Superior's precipitous decline was halted dramatically in October when the northern basin got a "monsoonal" drenching of twice its normal rainfall. The water supply in October was a new record maximum with some 20 per cent more than the previous record for that month. Instead of falling 4 cm, which it normally does in October, Superior's watermark rose 14 cm. On November 1, levels were 20 cm higher than they were six weeks before. However, by month's end, low supply conditions had returned to Lake Superior.
Rapidly shrinking water levels were a big challenge for commercial navigation. Freighters had to reduce their draft, thus lightening their loads by upwards of 10 per cent. Shallower waters also disrupted spawning grounds for fish and wetland wildlife, and created hardship for recreational boaters. Several marinas undertook dredging, while many private owners were left high and dry. As well, hydro-electric generation was reduced due to lower flows in the system.
Falling water levels in the Great Lakes have raised questions about the impact of climate change. Are we seeing much sooner what scientists forecasted would happen decades from now? There is no question that Superior's levels are being driven down in part by a lack of precipitation. In 2006, Lake Superior basin-wide precipitation was at its lowest since the mid-1920s, pushed down even further in early 2007. In recent years, the snowpack has been unusually scanty. Further, the lake is getting warmer, both above and below the surface. From 1979 to 2006, summer water temperatures rose about 2.5°C, twice as much as the overriding air. With half the winter ice cover found 30 years ago, more of Lake Superior's water is being lost to evaporation year-round. All this is setting up a pattern of "warmer water, less ice, more evaporation" that is leading to lower water levels.
- Date modified: