Multiscale hydrogeomorphic influences on bull trout (Salvelinus confluentus) spawning habitat

Abstract

We investigated multiscale hydrogeomorphic influences on the distribution and abundance of bull trout (Salvelinus confluentus) spawning in snowmelt-dominated streams of the upper Flathead River basin, northwestern Montana. Within our study reaches, bull trout tended to spawn in the finest available gravel substrates. Analysis of the mobility of these substrates, based on one-dimensional hydraulic modeling and calculation of dimensionless shear stresses, indicated that bed materials in spawning reaches would be mobilized at moderate (i.e., 2-year recurrence interval) high-flow conditions, although the asynchronous timing of the fall-winter egg incubation period and typical late spring -early summer snowmelt high flows in our study area may limit susceptibility to redd scour under current hydrologic regimes. Redd occurrence also tended to be associated with concave-up bedforms (pool tailouts) with downwelling intragravel flows. Streambed temperatures tracked stream water diurnal temperature cycles to a depth of at least 25 cm, averaging 6.1-8.1°C in different study reaches during the spawning period. Ground water provided thermal moderation of stream water for several high-density spawning reaches. Bull trout redds were more frequent in unconfined alluvial valley reaches (8.5 versus 5.0 redds·km −1 in confined valley reaches), which were strongly influenced by hyporheic and groundwater -stream water exchange. A considerable proportion of redds were patchily distributed in confined valley reaches, however, emphasizing the influence of local physical conditions in supporting bull trout spawning habitat. Moreover, narrowing or "bounding" of these alluvial valley segments did not appear to be important. Our results suggest that geomorphic, thermal, and hydrological factors influence bull trout spawning occurrence at multiple spatial scales.Résumé : Nous avons étudié les influences hydrogéomorphologiques à différentes échelles sur la répartition et l'abondance d'ombles à tête plate (Salvelinus confluentus) frayant dans des cours d'eau à prédominance d'eau de fonte de neige dans le bassin supérieur de la rivière Flathead, dans le nord-ouest du Montana. Dans les tronçons étudiés, les ombles à tête plate avant tendance à frayer dans les substrats disponibles constitués du gravier le plus fin. Si l'analyse de la mobilité de ces substrats, basée sur la modélisation hydraulique unidimensionnelle et le calcul des contraintes de cisaillement adimensionnelles, indique que les matériaux du lit des tronçons de frai seraient mobilisés dans des conditions de débit modéré (c.-à -d. période de retour de 2 ans) à fort, l'asynchronisme de la période automnale et hivernale d'incubation des oeufs et des crues de fonte de neige, qui se produisent typiquement de la fin du printemps au début de l'été dans la région à l'étude, pourrait limiter la vulnérabilité des nids de frai à l'affouillement dans les régimes hydrologiques actuels. La présence de nids de frai avait également tendance à être associée à des formes de fond concaves vers le hau...

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