The drainage of the Lake Ha!Ha! reservoir and downstream geomorphic impacts along Ha!Ha! River, Saguenay area, Quebec, Canada

On July 18-21, 1996, a severe rainstorm caused widespread flooding along th e north shore of the St, Lawrence River, southern Quebec, Canada, particula rly along rivers that drain the area just south of the Saguenay-Lake St. Je an region. At the Lake Ha!Ha! reservoir, inadequate available capacity to s pill during the storm at the outlet dam resulted in the overtopping and ero sion 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 i ts area from 8.1 to 4.7 km(2). Estimates of discharge range from 910 to 138 0 m(3) s(-1) at the site of the eroded dyke to 1080 to 1260 m(3) s(-1) at a location 27 km downstream (about 8 km above the mouth of the river). The u ncontrolled drainage of the Lake Ha!Ha! reservoir increased flooding along the lower 35 km of Ha!Ha! River where flooding was already in progress beca use of the rainstorm runoff. The flooding caused extensive geomorphic impac ts along the river. Long sections of the river (totalling 25 km) experience d significant widening (locally up to 280 m) and channel incision (locally up to 20 m) while two reaches (6 and 1.5 km long) experienced up to several metres of aggradation. In general, the slope of the valley was the most im portant variable affecting whether or not the energy of the flow was above or below the erosive threshold of the valley bottom. Locally, a permanent c hannel diversion now exists where the drainage divide between the main rive r course and a small ravine was overtopped and extensively eroded. Communit ies, infrastructure, and industry located along the river were extensively damaged by the flood waters. The effects of flooding along Ha!Ha! River dem onstrate that rivers on the Canadian Shield can undergo severe geomorphic c hanges caused by very high-magnitude flooding. (C) 1999 Elsevier Science B. V. All rights reserved.