Channel type and salmonid spawning distribution and abundance

Consideration of fundamental channel processes, together with map-based and field investigations, indicates that stream channel type influences salmon id spawning distributions across entire channel networks and salmonid abund ance within channel reaches. Our analysis suggests that salmonid spawning p atterns in mountain drainage basins of the Pacific Northwest are adapted to , among other things, the timing and depth of channel bed mobility. We hypo thesize that because the bed of pool-riffle and plane-bed reaches scours to a variable fraction of the thickness of alluvium, survival to emergence is favored by either burying eggs below the annual scour depth or avoiding eg g burial during times of likely bed mobility. Conversely, annual mobility o f all available spawning gravel in steeper step-pool and cascade channels f avors either adaptations that avoid egg burial during times of likely bed m obility or selection of protected microhabitats. Consistent with these expe ctations, we find that salmonid spawning distributions track channel slope distributions in several west-slope Pacific Northwest watersheds, implying that spatial differences in channel processes influence community structure in these rainfall-dominated drainage basins. More detailed field surveys c onfirm that different channel types host differential use by spawning salmo nids and reveal finer-scale influences of pool spacing on salmonid abundanc e.