Does Channel Incision Affect In-stream Habitat? Examining the Effects of Multiple Geomorphic Variables on Fish Habitat

Abstract

Incised river channels are dynamic components of fluvial systems, represent geomorphic degradation, and are encountered worldwide. Ecological effects of incision can be far-reaching, affecting habitat availability and channel processes. Although incision can reflect habitat degradation, some studies suggest that important in-stream habitats do not differ with the degree of incision. Therefore, we tested whether in-stream habitat variables that are important to imperiled fishes differ in river reaches with varying degrees of incision. Because incision (measured using entrenchment ratio) had no discernable effect on in-stream habitat characteristics (i.e., proportion fines, gravel, cobble, and macrophyte occurrence and length), we expanded our analysis to assess the effects of 29 additional geomorphic variables on in-stream habitat. These analyses indicated that bank height, bed mobility, D84, cross-sectional area, bankfull width, and wetted perimeter accounted for 42% of macrophyte occurrence and 64% of macrophyte length variance. Postflood surveys indicated that macrophyte occurrence on cobble declined as bank height and bed mobility increased, and sediment size decreased, suggesting that sediment size and bed mobility have a stronger influence on in-stream habitat than incision. Although channel incision often indicates environmental degradation, important aspects of habitat are not described by this measurement. Strategies that depend on incision to identify restoration sites may have limited habitat benefits in Southeastern Piedmont streams and rivers. Instead, landscape or shoal-scale restoration approaches that increase coarse sediment proportions may increase macrophyte occurrence, length, and persistence. Sediment budgets that identify coarse and fine sediment sources and transport may be useful to prioritize restoration approaches.