| Abstract/Description or Keywords |
A considerable body of research has been built in the past 50 years in order to understand salmon habitat and the influence of anthropogenic activities. Geomorphologists have played an important role in this research, having described the sedimentary conditions, and channel morphology requirements for productive salmonid populations (Bjornn and Reiser 1991; Kondolf and Wolman 1993; Montgomery et al. 1999). Much of this work has sought to understand the sediment generated from catchment disturbances such as logging and associated activities such as road construction on salmonids, their habitat, and their reproductive dynamics. Recent research has considered the role of salmon as geomorphic agents controlling their own habitat (Montgomery et al. 1996; Gottesfeld 1998; Gottesfeld et al. 2004). Far fewer investigations have attempted to quantitatively describe the effects of salmon as geomorphic agents and their role in mobilizing sediment. In the course of excavating their spawning redds, riverine salmonids modify the streambed morphology and move sediment downstream. What is most remarkable about the excavation by the fish is that the individual actions of high concentrations of spawners lead to sediment mobilization and to large-scale changes in the local bed topography and channel morphology. In streams with dense populations of salmon, the whole surface of spawning reaches may be modified (Montgomery et al. 1996; Peterson and Quinn 1996; Gottesfeld et al. 2004). At these sites, salmon bioturbation may be the dominant or subdominant agent of sediment transport and affect the composition, stratification, and fabric of the stream bed sediments (Kondolf et al. 1993; Montgomery et al. 1996; Gottesfeld et al. 2004). Since some populations of salmon home to the stream and sometimes to the reach in which they were hatched (Wagner 1969; Cramer 1981), they may have a profound cumulative effect over many years. The primary objective of the chapter is to quantitatively explore spatial scales of salmonid bioturbation bedforms in gravel-bed rivers. We briefly review some of the literature on spawning habitat and salmon redd excavation. Then, we examine spawning bedforms in river channels at two different scales, from smaller tributary systems where the size of the redd imprint relative to channel width is large to broad river channels where hundreds of thousands of spawners rework the substrate of a single reach. Two notable spawning groups in British Columbia that exemplify high rates of bioturbation at different scales are the early Stuart sockeye gottesfeld et al. salmon Oncorhynchus nerka of the Stuart-Takla drainage (smaller-scale channels) and the Chinook salmon O. tshawytscha of the Morice and Harrison rivers (major drainages). In the smaller streams, fish are able to influence the mobility of sediment to a great extent, and with sufficient spawners, they contribute increasingly to rates of transport relative to the mean annual flood. There are, in fact, cases where sediment transport attributable to spawning fish is greater than the nival flood event. Examining patterns of bioturbation in larger rivers, we review the available data and observations that describe some of the conditions particular to Chinook bioturbation and dune bedform evolution in larger rivers. These rivers show large-scale perturbation of the bed, suggesting that sediment transport in the bioturbated reaches is strongly conditioned by spawning activity. Bed load flux rates are more difficult to quantify in these larger reaches. The morphologic imprint of the fish is not washed out as in the smaller systems, and analysis of sequential air photos suggests that the development and complexity of the bedforms is linked with the annual number of returning spawners. |
