Linking Geomorphological and Ecological Responses in Restored Urban Pool-Riffle Streams

Abstract

Little attention has been focused on evaluating river restoration projects. Postproject assessments commonly identify improvements in biological or physical conditions, but opportunities for understanding the interactions between such processes are often missed. This study assesses the linkages between geomorphological and ecological responses in three stream channels in Austin, Texas, restored since 1998 with riffles and steps and riparian planting along graded banks. Prerestoration topographic surveys and data for habitat and macroinvertebrate characteristics enabled comparisons with postrestoration conditions in 2007. Results showed increased channel widths and depths, leading to larger cross-sectional areas and inferred lowered velocities and unit stream power. Improvements in habitats included greater bank stability, less channel alteration, and more diverse velocity-depth regimes. Changes in functional feeding groups of macroinvertebrate communities were reflected in metrics of the Benthic Index of Biotic Integrity, including greater percentages of grazers, filterers, and collector-gatherers in the restored streams. A multivariate statistical model, redundancy analysis, linked improvement in key ecological response variables (taxa richness, % EPT, % grazers, and % chironomids) to better conditions in habitats (lower embeddedness, greater epifaunal cover, greater riparian vegetative width, and more velocity-depth regimes). Key explanatory changes in physical characteristics were increased cross-sectional area and decreased average velocity, which were attributed to restoration designs. These results suggest that although the main restoration goals were to stabilize eroding channels, improvements in physical characteristics could nevertheless lead to positive ecological outcomes. These findings suggest the potential of integrated approaches to target both physical and biological improvements in future restoration projects