Weather: What's happening to Great Lakes water levels?

Wednesday, January 16, 2008

With levels at their lowest since observations began in 1918, it's beginning to look as if evaporation due to climate change may be the culprit

by HENRY HENGEVELD

During a mid-October family gathering, the conversation inevitably turned to the weather. After all, fall temperatures have been unusually warm, and it has been so dry.

My brother-in-law noted that the water level in his pond, which he had dug on his property adjacent to the Bay of Quinte some 40 years ago, was now the lowest he had ever seen. He solemnly declared that something unusual was going on! The same sentiment appears to have been echoed in many households and boardrooms around the Great Lakes this fall.

These observations were given legitimacy by U.S. and Canadian lake authorities. They reported that, in September, both Lake Superior and the lower Montreal Harbour - at opposite ends of the Great Lakes water basin - reached their lowest water levels since routine observations began in 1918. Lakes Huron and Michigan were at their lowest levels since the 1960s, while the other lakes in between were also all below normal levels.

By October, heavy rains in the upper lakes had raised Lake Superior levels by 16 centimetres, but still well below normal. The St. Lawrence River at Montreal continued at record low levels. With drier than normal conditions predicted for the Great Lakes basin for this winter, experts predict that these levels will remain well below normal until at least the spring run-off period.

The consequences of these low water levels went well beyond simple inconvenience to pleasure boat operators trying to reach their docks. During October, some water wells in land areas within the Great Lakes basin had begun to run dry. Navigation advisories to the shipping industry had to warn mariners to use extreme caution throughout the Seaway system, especially during periods of strong winds when water levels can fall significantly in a short period of time.

In response, many marine vessels began reducing their loads by 10 per cent. For many, the reduced load capacity meant the difference between a business profit and a net operating loss. Furthermore, observers noted that ships passing through the Seaway channels were stirring up more mud than usual from the lake and river bottoms. Environmental groups raised concerns that this may be re-introducing toxic chemicals long buried in the lake sediments back into our drinking water. Others expressed concern about the impact of dried up wetlands along the lake shores on waterfowl and other species.

There were, of course, the usual conspiracy theories to explain the low water levels, particularly in the upper lakes. After all, if we can find some to blame, then we can also sue for damages.

In particular, various interest groups in the upper lakes region argued that past dredging of the St. Clair River by U.S. marine engineers to increase Seaway shipping capacity had allowed the river bottom to erode and deepen, thus partially removing a natural plug that slowed the outflow of water from the upper Lakes into the lower Lakes. As a result, they suggest, more water is leaking out of Lakes Superior, Huron and Michigan through the Seaway and lowering water levels in the upper lakes.

Researchers have recently investigated this theory by dragging a camera behind a boat along the St. Clair River bottom. They found large masses of bedrock along the bottom that is resistant to erosion. While there is the possibility that the river bottom erosion has already occurred, leaving only bedrock, there is no evidence to support this theory. Furthermore, the available data indicates that water levels are also dropping in the lower lakes.

A more plausible and logical explanation is that the low lake levels are a result of changes over time in the balance between the input of water into the lakes through precipitation and runoff from land areas and the removal through evaporation.

While precipitation has been very low this summer, average rainfall in the entire basin has been close to normal over the past six years. Hence, reduced precipitation can help partially explain the rapid drop of the past six months, but not the longer-term trend of the past decade.

However, water and air temperatures have risen significantly. In Lake Superior, for example, water temperatures have risen by about 1 C per decade over the past 25 years - about twice the rate observed for air temperature. Meanwhile, largely because of the rise in temperature, the duration and extent of average winter lake ice cover has decreased dramatically - by about 50 per cent over the past century.

This extends the period of time when the surface water is exposed to the atmosphere.

There is also evidence that winds over Great Lakes have increased. All of these factors contribute to greater evaporation of surface waters. Since warmer air holds more moisture, much of the additional water vapour remains in the air to blow away with the wind, rather than falling on land nearby and re-entering the lakes as runoff. In other words, the added water sink hole is far more likely to be atmospheric transport than a faster flowing Seaway system.

Climatologists caution that, during the past century, water levels in the Great Lakes have varied by over a metre in the past. Hence, recent trends could simply be part of a long-term natural fluctuation. However, the trends are also consistent with that expected due to global climate change. Thus, experts suggest that the record low levels of Lake Superior may be an early warning of what climate and hydrological modellers have been projecting for quite some time.

The International Joint Commission, an advisory body of Canadian and American experts which advises both U.S. and Canadian governments on water policies across the border, has recently launched a major study to investigate this possible linkage between lake level trends and climate change. They will also seek to improve on past projections for lake response to warmer climates expected in the decades to come.

Unfortunately, it'll take another four years before we hear the results. BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada