Geomorphic effects of flooding along reaches of selected rivers in the Saguenay Region, Quebec, July 1996

A severe rainstorm from July 18 to 21, 1996, caused widespread flooding along many rivers in southern Quebec, particularly in the Lac-Saint-Jean-Chicoutimi area, along north-flowing tributaries of the Riviere Saguenay. Along study reaches located on the Riviere aux Sables, Riviere Chicoutimi, Riviere du Moulin and Riviere A Mars, the fluvial geomorphic effects of flooding varied considerably, reflecting differing channel morphologies (alluvial, non-alluvial and bedrock), flow energy, and the interaction of floodwaters and infrastructure. Catastrophic channel widening and floodplain reworking occurred along the Riviere a Mars study reach, transforming the river from a meandering to braided planform. Consistent with this, unit stream power of the flood flow exceeded the minimum erosive threshold (300 Wm(-2)) along most of the study reach. Morphological and empirical evidence indicates that the pre-flood Riviere A Mars channel exhibited a transitional planform, and thus was vulnerable to a planform transformation during an extreme flood. The most significant geomorphic, effects along the Riviere aux Sables and Riviere Chicoutimi study reaches occurred at a number of run-of-the-river dams. Floodwaters overtopped four dams and eroded deeply into unconsolidated sediments adjacent to the dams, forming now channels that captured the flow of the river. Floodwaters also overtopped a fifth dam, scouring overburden and road beds, and damaging and destroying buildings within an urban subdivision. Elsewhere along these two rivers, as well as along the Riviere du Moulin study reach, there were localized erosional problems, particularly at bridges, but the geomorphic effects generally were limited or negligible, reflecting either low unit stream power or resistant substrates.