Bolstering the Banks of the St. Lawrence

Waves caused by wind and passing ships, fluctuations in water level, ice and currents are eating away the banks of the St. Lawrence River, with some areas receding at a rate of up to three metres a year. Environment Canada scientists are working to identify, stabilize and restore the most threatened areas before they disappear and, in the process, to preserve vital bird and wildlife habitat along the river's edge.

Studies conducted three years ago by Environment Canada and its partners under the St. Lawrence Vision 2000 program were the first to precisely document the magnitude of the problem along the length of the waterway. Using recent surveys and historical aerial photographs, they discovered that 25 per cent of the 1 500 kilometres of shoreline between Cornwall and Quebec City show signs of active erosion. It is estimated that 2 000 hectares of habitat have been lost since the 1960s, when unusually high water levels weakened the integrity of the banks.

Most profoundly affected is the area between Montréal and Lac Saint-Pierre, west of Trois-Rivières, where 50 per cent of the shoreline is receding and an estimated 1 500 hectares of island shore habitat have been lost. The problem is due, in part, to the fact that this stretch of the river is relatively narrow, and the shoreline is therefore in closer proximity to the shipping lane, which sees more than 15 000 passages a year. The region also has almost three times as many islands as other parts of the river, so more shoreline is affected.

Although erosion is a natural process, its rapid acceleration—caused, in part by human activity—threatens to destroy important nesting sites for waterfowl and spawning areas for fish, and could affect other bank-dwelling wildlife such as muskrats, frogs, turtles and songbirds. While some shorelines have been stabilized using inert concrete and stone embankments in the past, these inert methods do not favour vegetation growth, and the resulting shoreline has little ecological or aesthetic value.

To address the problem, Environment Canada has piloted several natural restoration techniques at key sites along the St. Lawrence. One is Îles de la Paix National Wildlife Area, on the south shore of Montréal in Lac Saint-Louis. Home to a dozen threatened or vulnerable species, this sensitive 50-hectare archipelago is losing its shoreline at a rate of 2.2 hectares per year, a clip that could erase it from the map within 20 years.

In an effort to slow the process and reverse existing damage, scientists tried stabilizing an embankment by filling horizontal trenches with rock-filled metal baskets (gabions) and bundles of branches, covering the area with branch mats topped with wire mesh, adding earth and then planting trees and other vegetation. Although the method met with some success, water destroyed some of the lower-lying vegetation and degraded the gabions, enabling some of the rocks to wash away. A similar approach, using larger rocks, was employed successfully at nearby Varennes—one of the most important waterfowl nesting grounds on the St. Lawrence.

A different technique was tested at Îles de Contrecoeur National Wildlife Area, east of Montréal, where barriers formed by material that was dredged during the deepening of the waterway are rapidly eroding and exposing the string of 27 smaller islands to the ravages of the river. Rows of cedar logs and branch bundles were used to shore an eroding embankment and the area was covered with earth and living shrubs. The method was later revised to bring the logs nearer to the base of the embankment.

A fourth project attempted to introduce vegetation to 100 metres of stone-lined embankment on the Beauport Strands, east of Quebec City, to determine whether or not the many existing areas of this type can be restored to useful production. By filling spaces between the rocks with different types of mulch and planting various species of seedlings, scientists determined that bark mulch was less prone to sifting down through the rocks than shredded leaves or branch shavings were, and that Canadian alder was the most hardy candidate for such plantings—with a 50-per-cent survival rate. Pebbles were recommended to further reduce the problem of the mulch washing out from below.

Over the past two years, Environment Canada has financed 71 stabilization projects along the St. Lawrence, totalling $7.3 million, through its own EcoAction 2000 and the St. Lawrence Action Plan's Community Interaction programs—both of which promote community-based environmental efforts. With costs as high as $1 000 per metre for some shoreline restoration projects, scientists are now trying to determine exactly how rapidly each section of bank is eroding and the potential impact this will have on wildlife habitat. This work—which will take two to three years to complete—will be useful in determining priorities for future action in the effort to save the banks of the St. Lawrence.

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