THE PRECISE AND PARSIMONIOUS APPROACH TO PHYSICAL AQUATIC HABITAT REHABILITATION IN LARGE-RIVER MANAGEMENT

Physical habitat in rivers and streams has been the focus of substantial interdisciplinary research, motivated by the need to understand how alterations to aquatic habitats affect aquatic organisms and how management actions can be applied to rehabilitate degraded systems. While holistic approaches can be usefully applied to some smaller stream systems – those for which restoration of flow, sediment, and wood regimes is sufficient to restore form and processes – rivers with substantial socio-economic services present acute tradeoffs that compel more reductionist approaches that focus on the precise habitat requirements of aquatic organisms. Often such efforts are applied to specific species at risk rather than ecosystems or communities. Rather than being guided by conditions in natural, unimpaired rivers, reductionist approaches typically seek to provide precisely and parsimoniously the exact physical habitat needed by specific organisms for critical life-stage transitions. Compared to holistic restoration, the parsimonious provision of specific habitat conditions allows for greater accommodation of socio-economic services and increases ability to maximize benefit:cost ratios of management actions. Such reductionist approaches require precise and accurate understanding of 1) the biology of aquatic organisms of interest and 2) process linkages from river management to habitat requirements. Because fluvial geomorphology quantifies the physical processes that link many management actions to habitats and biotic responses, geomorphic understanding is typically a necessary – if not sufficient – piece of informed river management. We present lessons learned from research on recovery of an endangered fish (pallid sturgeon) in large rivers of the Midwest and restoration of native mussel populations in Ozark and Appalachian rivers. The case studies illustrate the need for engaged interdisciplinary research and enlightened engineering when holistic restoration is not tenable.