Salmonid Spawning Habitat in Rivers: Physical Controls, Biological Responses, and Approaches to Remediation
Factors Controlling Availability of Spawning Habitat for Salmonids at the Basin Scale
Daniel J. Miller, Kelly Burnett, and Lee Benda
Measures of habitat attributes are well defined at the unit (1–10 m) and reach (10–1,000 m) scales, but observations at these scales can miss interactions over larger spatial extents (Fausch et al. 2002). Attributes that characterize upstream basin properties can exhibit relationships to fish abundance unresolved at the reach scale (Feist et al. 2003), so that aquatic habitats are increasingly being examined in terms of basin-scale metrics (Hughes et al. 2006). Basin-scale attributes found to correlate with spawner abundance include road density (Baxter et al. 1999), the proportion of basin area: with hillslope gradients below a certain threshold, in certain forest types, in certain land uses, and in certain rock types (Pess et al. 2002; Feist et al. 2003; Steel et al. 2004). Such attributes serve as indicators of process types and rates in a basin, gauging levels of erosion and sediment transport and recruitment of large wood to channels.
Here, we review studies describing geomorphic processes affecting physical habitat, particularly salmonid spawning habitat as influenced by the supply, movement, and storage of coarse sediment through a channel network. Our examples are drawn primarily from the mountainous terrain of the Pacific Northwestern United States. Although not exhaustive, we examine processes of sediment supply and transport that operate in all rivers; the implications for spawning habitat depend on the spatial and temporal distribution of process rates, which vary both locally and regionally. Such a framework can help quantify these variations and guide development of analysis tools and collection of data to characterize basin-wide controls on habitat formation and alteration.
Habitat requirements for salmonid spawning are fairly specific: a stable channel formed in clean gravel of the appropriate size with abundant hyporheic flow (Kondolf 2000). The prerequisites for developing such habitat are also fairly specific: a valley-floor and channel configuration that promotes alluvial deposition of gravels, conditions that exist within a limited range of channel sizes and gradients. Once this range is determined, it is relatively straightforward to delineate the extent of a river system potentially suitable for habitat formation (Lunetta et al. 1997; Buffington et al. 2004; Burnett et al. 2007). This is a useful exercise: it lays out the stage on which watershed processes act to form the habitats required for successful spawning. This can be a rather large stage, with many simultaneous, interacting scenes and numerous actors.