Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Spawning Habitat Rehabilitation: Advances in Analysis Tools
Gregory B. Pasternack
River restoration is one of the most rapidly growing professional practices in hydrology worldwide today. According to a recent national synthesis, no less than 37,000 river restoration projects have been conducted in the United States alone (Bernhardt et al. 2005). Such projects range in size and cost from localized instream habitat improvement (~US$20,000 per project) exemplified by spawning gravel placement below dams (Merz and Setka 2004) to large-scale land acquisitions (~$812,000) such as the buyback of sugar plantations in the Kissimmee basin of Florida (Loftin 1991).
Like other civil construction activities, river restoration should involve the engineering application of accepted scientific principles. However, there is mounting evidence that many guiding principles used in restoration design and construction are either too uncertain or invalid to yield the expected outcomes when applied to local project sites (Sear 1994; Thompson and Stull 2002). Classic hydrologic and geomorphic research sought to discover the general nature of how rivers behave (Wolman and Miller 1960; Leopold et al. 1964; Williams 1978). In contrast, active river restoration involves site-specific reinitialization of linked hydrogeomorphic processes and ecological functions. Key interactions are highly nonlinear with counterintuitive feedbacks. Thus, there should not be an expectation that first-order general relations should apply to or sufficiently characterize any given site. Moreover, the actions required to restore rivers may not involve a simple reversal of those responsible for degradation, due to the concept of biophysical hysteresis (Brooks et al. 2006). Concerns over river restoration are mounting (Palmer et al. 2005; Hughes et al. 2005), but river restoration efforts will continue and new scientific advances need to be codified in restoration methods.
Spawning habitat rehabilitation (SHR) is a form of river restoration in which improving a key biological indicator of ecosystem functionality in rivers—available quantity of high-quality salmon spawning habitat—is the focus of active intervention into river processes and channel morphology (Wheaton et al. 2004a; Roni 2005). Spawning habitat rehabilitation is motivated by significant interdecadal downward trends in anadromous fish populations whose spawning habitats have been cut off and severely depleted by dams, sand accumulation (Greig et al. 2005), gravel mining, and other in-channel human activities to the point that spawning habitat quality limits population size (Nehlsen et al. 1991; Moyle 1994; Moyle and Randall 1998; Yoshiyama et al. 2000). Because SHR is often performed on heavily regulated streams whose flow regime may never be naturalized, it may serve as a final measure to stem local wild population declines beyond the point of no return (Hendry et al. 2003).