The River Machine: A Template for fish movement and habitat, fluvial geomorphology, fluid dynamics, and biogeochemical

Abstract

Dynamics of many large floodplain rivers are dominated by the flood pulse. The highkinetic energy of the erosive flows associated with the flood pulse form and reform the riverchannel. In general, the flood pulse supports the immense abundance and diversity of river lifeby transporting nutrients and organic matter into backwaters in spring, supporting primary andsecondary production during the summer, and redistributing these products to channels as waterlevels recede. Both North American and South American fluvial dependent large river fishesexhibit complex, system-level longitudinal and/or lateral movements across life stages that allowthem to exploit flood pulse driven spatial heterogeneity and seasonal connectivity to feed,reproduce, and avoid harsh conditions. We argue that two hydraulic variables, the magnitudes ofvelocity and the spatial velocity gradient, are necessary and sufficient to both understand fish“hydro-navigation” as well as explain patterns in biogeochemistry and fluvial geomorphologyand thereby create a new conceptual framework for large flood plain rivers integrating fluiddynamics, channel morphology, biogeochemical cycling, and important elements of fish ecology.We illustrate the framework using summary data from the São Francisco River, Brazil thatcontains sub basins possessing different levels of impact and also from the Lower Paraná River(Argentina) where natural processes can still be studied. We believe the framework is animportant element of large river restoration because it directly links the unique physical andchemical processes of large floodplain rivers to life requirements important to fishes and otherbiota.