Evaluation of pool‐riffle naturalization structures on habitat complexity and the fish community in an urban Illinois stream

Abstract Urbanization and its associated stressors such as flow alteration, channel modification and poor water quality is a leading cause of ecological degradation to rivers and streams. Driven by public concern to address this issue, there has been a dramatic increase in urban restoration projects since 1990 using in‐stream structures. Attempts at restoring the ecological condition of urban streams using structures have produced varied results, but projects do not often meet planned ecological goals. A major challenge to improving the ecological health of urban streams is to better understand how to incorporate ecological assessments into a ‘restoration’ design framework with reasonable expectations for ecological recovery. A naturalization design framework was used in a project on a 0.62‐km reach of the North Branch of the Chicago River in Northbrook, Illinois. Initial surveys of channel morphology, habitat and biota identified poor pool‐riffle bed structure and fish biodiversity, which became the basis for research and development of a pool‐riffle structure specifically designed for constrained, low‐gradient channels. Habitat and fish surveys were conducted pre‐ and post‐construction. The project improved mesohabitat structure, and fish abundance, and biomass and diversity were greater for 2 years following construction (2002–2003) compared to 3 years prior to construction (1999–2001). However, the improved fish metrics were in the low range when compared to rural streams in the same ecoregion, and the fish community consisted primarily of tolerant, slow‐water species. Absent were intolerant and riffle dwelling species, such as insectivorous cyprinids and darters. Assessment of pre‐ and post‐project ecological condition and the use of species information provided a basis for ecologically informed design and expanded our understanding of the limitations to restoring urban streams. Copyright © 2007 John Wiley & Sons, Ltd.