Comparison of mainstem spawning habitats for two populations of fall chinook salmon in the Columbia River basin

Extensive hydroelectric development in the Columbia River system has elimin ated most mainstem riverine habitat available for spawning by fall chinook salmon (Oncorhynchus tshawytscha). The two remaining populations, Hanford R each, Columbia River and Hells Canyon Reach, Snake River, are separated geo graphically and their status is markedly different. Annual escapements to H anford Reach have averaged approximately 80000 adults, while the Snake Rive r run size has declined to < 1500 adults over the past 10 years. We compare d their spawning habitat characteristics over a range of measurement scales , as a means to identify strategies for rebuilding the weak Snake River pop ulation. Physical habitat characteristics of redds were similar for both st udy areas. Redd locations were correlated with channel characteristics, suc h as braiding and sinuosity. Several differences between the two spawning a reas were identified at the watershed scale: the Hells Canyon Reach had a m uch steeper longitudinal gradient, was largely confined by bedrock, and had a more variable flow regime. These features are controlling variables that operate at the reach-scale to limit the availability and size of substrate and other conditions that influence egg deposition and incubation survival . Geomorphological characteristics of the two study sites are sufficiently different to indicate that the production potential of the Hells Canyon Rea ch population is markedly lower than that of the Hanford Reach population. Copyright (C) 2000 John Wiley & Sons, Ltd.