CHANNEL MORPHOLOGY AND LATERAL STABILITY - EFFECTS ON DISTRIBUTION OFSPAWNING AND REARING HABITAT FOR ATLANTIC SALMON IN A WANDERING COBBLE-BED RIVER

Pronounced downstream variations in channel morphology in the wanderin g, gravel-cobble Nouvelle River, Quebec, provided an opportunity to as sess certain effects of channel planform and stability on rearing and spawning habitat for Atlantic salmon (Salmo salar). At summer low flow s, weighted useable area per unit channel length differed significantl y among five reaches of contrasting morphology with identical discharg e regimes. A braid-like reach, dominated by a wide, dissected midstrea m bar, offered three to five times more potential habitat for juvenile s (21 m usable width for fry, 12 m for parr) than two sharply curved r eaches with minor backchannels (4 m for both fry and parr) and two to three times more potential habitat than two moderately curved reaches. Fining of potential spawning riffles downstream from eroding cutbanks was not detected, in five unstable river reaches even where erosion r ates reached 10 m/year, and the percentage sand content of eroding ban ks was five times that of instream gravels. This finding challenges th e assumption that large local inputs of sand necessarily cause fining of instream spawning gravels, and suggests that hydraulic forces in a moderately powerful river can be quite efficient in preventing the bui ldup of excess fines in midchannel riffle habitat.