Linking spatial patterns of bed surface texture, bed mobility, and channel hydraulics in a mountain stream to potential spawning substrate for small resident trout

• Spawning substrate availability and disturbance in a trout-bearing mountain stream
• Two “domains”: limited availability-high disturbance vs. abundant-low disturbance
• Stable bed, disturbance risk associated mostly with fine sediment in wake deposits
• Rapid transition between the domains as bed becomes finer: substrate sensitivity
• Small-scale geomorphic processes and their relative spatial configuration important

In this paper we examined spatial patterns of bed texture, bed mobility, and channel hydraulics in four reaches of a mountain stream with moderate gradient (< 0.02) and linked them to availability and disturbance of potential spawning substrate for small resident cutthroat trout (body size 200 mm). Within-reach spatial distribution and extent of potential substrate and disturbance risk enabled us to identify between-reach differences, which pointed to existence of two distinct domains within the study area. In two cobble–gravel reaches bed was generally too coarse for spawning and potential substrate appeared to be limited to small areas protected by flow obstructions. These hydraulically sheltered sites promoted accumulation of large quantities of fine sediment that seemed to pose high disturbance risk to the associated substrate patches. In contrast, general bed fining resulted in development of extensive areas of spawning substrate, unaffected by excess fine sediment deposition, in the unobstructed portions of gravel-dominated reaches. In all study reaches bed mobility and risk of scour disturbance was generally limited, even though its significance was sensitive to the adopted value of eggs burial depth. Abrupt transition between the two contrasting domains in response to relatively modest changes in channel texture led us to hypothesize that small-bodied salmonids in similar mountain streams may be vulnerable to habitat fragmentation that prevent fish movement between reaches but also to climate and land use changes that alter inputs of water and sediment into the channels. Overall, our within-reach analyses illustrated that between-reach differences in potential substrate availability and disturbance risk may be strongly dependent on small-scale hydro-geomorphic processes and their configuration in space relative to one another.