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TitleFlooding from the July 18-21, 1996 rainstorm in the Saguenay area, Quebec: fluvial geomorphic effects and slope stability along selected major river reaches
AuthorBrooks, G R; Lawrence, D E; Fung, K; Bégin, C; Perret, D
SourceGeological Survey of Canada, Open File 3498, 1997, 178 pages (1 sheet), (Open Access)
PublisherNatural Resources Canada
Documentopen file
Mediapaper; on-line; digital
File formatpdf
NTS22D/06SE; 22D/06NE; 22D/07SW; 22D/07NW
AreaChicoutimi; Rivière Saguenay; Rivière aux Sables; Rivière Chicoutimi; Rivière du Moulin; Rivière des Ha!Ha!; Lac Ha!Ha! reservoir; Riviere à Mars
Lat/Long WENS -71.2500 -70.7500 48.5000 48.2500
Subjectssurficial geology/geomorphology; environmental geology; hydrogeology; surface waters; rivers; floods; erosion; landslides; landforms; slope stability; environmental impacts; environmental studies; fluvial deposits; drainage; transportation; dams; natural hazards; infrastructure; Phanerozoic; Cenozoic; Quaternary
Illustrationslocation maps; tables; sketch maps; histograms; hydrographs; photographs; aerial photographs; cross-sections; profiles; graphs; satellite imagery
Released1997 10 01; 2017 02 23
AbstractA major storm system stalled over the mouth of the St. Lawrence River from July 18-21, 1996 and dropped record amounts of rain causing widespread flooding in southern Quebec. Flooding was particularly severe along river systems south of the Saguenay-Lac Saint Jean area, where rainfall in excess of 200 mm fell within a 36 hour period.
The flooding caused severe geomorphic effects along five rivers in the area. These effects varied considerably both between rivers and from reach to reach along the same river. Along the Rivière aux Sables and Chicoutimi Rivière, the flooding caused dramatic, yet localized, lateral bank erosion and incision adjacent to a number of small dams creating deep channels which carried the post-flood flow around the dams. Elsewhere, negligible to moderate bank erosion and sedimentation occurred along the river banks and floodplains. Major lateral channel erosion and avulsions occurred along the lower 10 km of Rivière à Mars causing extensive destruction of the floodplain and erosion of terraces along the valley bottom. Negligible to minor geomorphic changes occurred along the lower 17 km of Rivière du Moulin; there being little evidence of the recent flood along long reaches of the river valley.
The worst flooding in the region occurred along Rivière des Ha!Ha! where the overtopping and erosion of an earthfill dyke caused the drainage of Lac Ha!Ha! reservoir. The geomorphic effects of the flooding varied downstream along the alluvial sections of the river. In general, wide, relatively gently-sloped reaches of valley bottom experienced considerable accretion of sand and silt (up to several metres) and negligible to minor erosion. Major channel incision and moderate channel widening occurred along confined, relatively steeply-sloped sections of valley. Along similarly sloped, but much wider reaches of valley, major channel widening with minor to negligible incision occurred, resulting in the reworking of large areas of the floodplain and the erosion of the valleysides.
Associated with the flooding, a number of landslides occurred along the five study rivers. Most failures were shallow and of limited extent, although two retrogressive slides caused damage to infrastructure and buildings. Triggering mechanisms for the landslides included bank erosion, saturation of the ground from the rainfall, drawdown of reservoirs and recession of flood waves; or a combination of the above. Overall, landsliding along the river banks was a relative minor part of the flood impacts.