Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Bed Disturbance Processes and the Physical Mechanisms of Scour in Salmonid Spawning Habitat
Salmonid embryo survival can be influenced by scour and related physical changes to the streambed because of a lengthy intragravel residence time during seasons when bed mobility is likely. Scour-induced mortality to this phase of the salmonid life cycle could influence population size if the substrate is excavated to, or mobilized at, the depth of developing embryos in a large proportion of redds (McNeil 1966; Seegrist and Gard 1972; Erman et al. 1988; Lisle 1989; Kondolf et al. 1991). In addition, as described in the next chapter (Sear et al. 2008, this volume), scour also facilitates fine sediment intrusion below the active bed layer and may thus influence survival indirectly because increased concentrations of fine sediments in redds are linked to decreased survival to emergence (STE; Chapman 1988; Lisle 1989; Scrivener and Brownlee 1989).
This chapter reviews the physical processes influencing scour depth in salmonid spawning habitat. Future advances in research of the consequences of scour for salmonid reproductive strategies and intragravel survival will depend on improving our ability to characterize and quantitatively model the mechanics of bedload transport and scour. Topics also include measurement issues, some potential linkages to land and water use, and the relation of specific spawning behavior attributes with possible selective pressures imposed by the mechanics of scour. The chapter begins with a review of mechanics and then evaluates various study results in the context of scour processes. In presenting a concise synthesis of physical processes and selected corresponding direct biological linkages for a collective audience of biologists, physical scientists, and engineers, it is hoped that this will help stimulate future research hypotheses that reflect an integrated, cross-disciplinary understanding of the nature of the scour problem.