Definition of Channel and Riparian Dynamics across a Disturbance Continuum: Implications for Salmon Restoration

Abstract

A spatially explicit watershed and riparian habitat model was developed for quantifying site characteristics of riparian habitats of the lower Cedar River, Washington. The spatial complexity and distribution of combined habitat and anthropogenic landscape features were used to define habitat "indices" of the relative quality of riparian habitats. Patches of contiguous grid cells were measured in terms of their locations, sizes, and relative degree of fragmentation to estimate riparian restoration opportunities in the watershed. One key area was impacted by a channel-damming landslide on February 28, 2001 when a magnitude 6.8 earthquake hit the region, causing the river to occupy an existing floodplain side channel. The channel and floodplain ecosystem responded rapidly until the channel's geomorphology settled into a short-term dynamic equilibrium according to LeChatelier's General Law. Large trees recruited to the channel facilitated lateral channel expansion as well as bed topographic variability. One year of pre-disturbance data was compared with three years of post-disturbance data in a spatially explicit analysis of the complexity of channel form and bed surface elevations. The landslide disturbance changed reach-scale habitat conditions and distributions of sockeye salmon (Oncorhynchus nerka). Annual changes in river habitat conditions and spawning distributions before and after the disturbance were evaluated with an exhaustive CHAID (chi-squared automatic interaction detection model. Before the landslide, the spatial and temporal patterns of sockeye distributions were heavily influenced by spawning locations of parental generations. Sockeye densities were highest in the upper-most river reaches where anthropogenic influences were least apparent. After the landslide, habitat complexity and fish densities increased in the lower river reaches that were directly affected by the landslide disturbance. The CHAID model analyses indicate fluvial habitat complexity, river discharge characteristics, and the fish density of respective parent generations were significant (P < 0.001), and explained spawning distributions by reach. Analyses of how changes in habitat complexity influence salmon populations at the reach-scale, inform questions of the magnitude and intensity of disturbance necessary to restore habitat functions of river landscapes that are vital to restoring imperiled salmon populations.