St. Lawrence has its Ups and Downs

The water levels of the St. Lawrence have been monitored and recorded since the mid-1800s. These records show long-term cycles of peaks and valleys lasting 15 to 35 years, with a two-metre difference in average levels between the highest and lowest years. The river's lowest levels occurred in the 1930s and 1960s, and the trend is once again at the bottom of the curve. These levels could be severely affected by climate change, because higher temperatures would increase evaporation and longer periods of drought would decrease flow.

To determine just how severe this effect could be, scientists at Environment Canada's St. Lawrence Centre and in the Meteorological Service of Canada are working with the Quebec government and universities to study the physical, chemical and biological components of the St. Lawrence River ecosystem. Their research involves everything from laboratory work to taking extensive samples of the river, shoreline and surrounding wetlands.

Although the sampling is still in its early stages, several trends are emerging. Extreme low water levels observed in 1999 resulted in a difference in the surface area of available habitats, causing an invasion of terrestrial, opportunistic plants into the dried-out wetlands. To the untrained eye, there may not have seemed much of a difference. For example, certain sections of riverbank seemed quite green, even when water levels were low. However, the question is of quality versus quantity. Are the plants in good variety, are they native plants, or is there an increase in the alien species and plants that don't provide much by way of food and cover for wildlife?

Animals, microorganisms, oxygen levels in the water, and the richness of the ecosystem itself are all affected by lower water levels. In the spring, for instance, less water is an advantage to duck populations because there is less risk of nests being flooded. But if the water stays low, there are no longer deep pools within reed beds where the ducks can feed, forcing them out of sheltered areas to find food. In the open water ducklings become more vulnerable to predators and disturbance by pleasure boaters.

St. Lawrence Centre biologists taking water-level measurements in a dried-up marsh on the St. Lawrence River near Boucherville, Quebec, in August 1999. Under normal conditions, the water would come up to waist level.

The data collected through field sampling are fed into a numerical model of the St. Lawrence River that was developed by Environment Canada in cooperation with the National Institute for Scientific Research and other partners. The model, which concentrates on the region from Cornwall to Trois Rivières, can be used to assess the impacts of different water-level scenarios in which the river physics and chemistry change. In essence, it uses physical aspects of the habitat, such as water depth, currents, waves and sediment, to predict the potential impact on the flora and fauna. A greater knowledge of the physics of the river also enables an informed response in the case of an oil spill.

Integrating biological information from field sampling with hydrological modelling will lead to a better understanding of the environmental challenges facing the St. Lawrence River ecosystem, and provide a framework for measures to protect this valued and integral part of Canada's natural landscape.

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