Multi-scale hydrogeomorphic influences on bull trout spawning habitat in snowmelt-dominated headwater streams

I investigated relationships between geomorphology, hydrogeology, and bull trout (Salvelinus confluentus) redd occurrence and density at multiple spatial scales in gravel-bed, pool-riffle, snowmelt dominated headwater streams of northwestern Montana. Subreach redd occurrence tended to be associated with the finest available textural facies. In subreach streambed sections hosting bull trout redds, redd density was significantly (at α=0.05) positively related to bankfull Shields stress (τ*bf, p=0.04) and bankfull Shields stress adjusted for grain stress only (τ**bf, p=0.02). In stream reaches hosting bull trout redds, reach-average redd density was significantly positively related to reach-average τ**bf (p=0.02) and reach-average streambed grain size (D16, p=0.01; D50, p=0.02, D84, p=0.02). Spawning reaches exhibited high streambed horizontal and vertical hydraulic conductivities, and streambed temperatures were dominated by stream water diurnal cycles to a depth of at least 25 cm. Groundwater provided substantial thermal moderation of stream water for multiple high density spawning reaches. At the valley-scale, redd occurrence tended to be associated with unconfined alluvial valleys. Many previous studies highlight the thermal sensitivity of bull trout. My spawning gravel competence results indicate that a shift in the timing of high flows could increase the likelihood of redd scour during the bull trout egg incubation period