Shellberg, Jeffrey. 2002. Hydrologic, geomorphic, and biologic influences on redd scour in bull char (Salvelinus confluentus) spawning streams. M.S.
A variety of physical factors besides temperature may influence bull char (Salvelinus confluentus) spawning distribution in western Washington. Factors, studied at various hierarchical scales, included watershed flow regime, sediment transport regime, channel type, and habitat unit type. It was hypothesized that fall spawning bull char with shallow to moderate egg burial depths and long incubation periods are prone to bedload redd scour during winter rain and rain-on-snow events in western Washington. The contrasting hypothesis that bull char select sites at multiple scales that provide refuge against bedload disturbance also was investigated.
Flow regimes in 85 bull char watersheds in Washington and the Pacific Northwest were defined using various hydrologic metrics. Watersheds with a high probability of peak flows and subsequent bedload transport during the bull char incubation period had the highest mean monthly discharge during incubation, a mean peak flow timing during incubation, a high monthly maximum seasonality, a steep flow duration curve, and a high annual coefficient of variation. Watersheds with these characteristics typically are dominated by precipitation as rain with little permanent snow cover except at the highest elevations. Despite the nature of flow regimes in these watersheds, many still contain(ed) bull char. In contrast, snow dominated watersheds had low probabilities of peak flows during the char incubation period. Many watersheds with moderate probabilities of peak flows and sediment transport during the incubation period existed between these two extremes. In these basins, changes in flow regimes due to climate change and/or land-use may influence the suitability of watershed hydrology for bull char spawning.
In western Washington, three main watersheds with moderate to high probabilities of peak flows during the char incubation period were chosen to study the interactions of flow regime, channel type, habitat unit type, selected habitat, and redd scour. Numerous site selection variables were measured to define suitable spawning habitat and compare study sites both within and between watersheds. The variables included depth, velocity, substrate size, distance to cover, distance to bank, and vertical hydraulic gradient, in addition to temperature regime, channel type, and habitat unit type. These variables were used to determine differences between selected redd and study sites and non-selected (unused) study sites to ensure the applicability of scour data collected from non-selected sites. Resulting data adequately differentiated and defined potential bull char spawning habitat within each watershed, but varied among watersheds according to channel and habitat types. Many non-selected study sites appeared to be unsuitable as prime spawning locations for bull char.
Bedload scour and channel topographic changes were measured using 152 scour monitors and 42 cross-sections in the three watersheds. Habitat selection by bull char influenced their exposure to potential scouring flow events. At a local scale, bedload scour depths to egg pocket depths in selected redd sites varied significantly between side-channel, protected main channel, and unprotected main channel redd sites. Unprotected gravel patches in simplified (lack of roughness) channel morphologies with moderate to high gradients were most susceptible to deep scour. Low gradient channel types appeared to provide moderate-flow scour protection through bed surface armor layers and a lack of sediment transport imbalances.
Implications of this study to basin wide bull char populations however are inconclusive, as a population level habitat use or mortality analysis was not conducted. Study data indicate that the distribution and availability of diverse fluvial spawning habitat might be able to offset potentially harsh flow regimes. Habitat rehabilitation strategies in hydrologically marginal habitat should focus on providing complex refugia habitat and reduced winter scour in spawning gravel.
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