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On July 18–21, 1996, a severe rainstorm caused widespread flooding along the north shore of the St. Lawrence River, southern Quebec, Canada, particularly along rivers that drain the area just south of the Saguenay–Lake St. Jean region. At the Lake Ha!Ha! reservoir, inadequate available capacity to spill during the storm at the outlet dam resulted in the overtopping and erosion of a nearby earthfill saddle dyke. A new outlet formed at the site of the dyke and drained the reservoir over a period of many hours decreasing its area from 8.1 to 4.7 km2. Estimates of discharge range from 910 to 1380 m3 s−1 at the site of the eroded dyke to 1080 to 1260 m3 s−1 at a location 27 km downstream (about 8 km above the mouth of the river). The uncontrolled drainage of the Lake Ha!Ha! reservoir increased flooding along the lower 35 km of Ha!Ha! River where flooding was already in progress because of the rainstorm runoff. The flooding caused extensive geomorphic impacts along the river. Long sections of the river (totalling 25 km) experienced significant widening (locally up to 280 m) and channel incision (locally up to 20 m) while two reaches (6 and 4.5 km long) experienced up to several metres of aggradation. In general, the slope of the valley was the most important variable affecting whether or not the energy of the flow was above or below the erosive threshold of the valley bottom. Locally, a permanent channel diversion now exists where the drainage divide between the main river course and a small ravine was overtopped and extensively eroded. Communities, infrastructure, and industry located along the river were extensively damaged by the flood waters. The effects of flooding along Ha!Ha! River demonstrate that rivers on the Canadian Shield can undergo severe geomorphic changes caused by very high-magnitude flooding. |
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