Flow and Form: Large-River Alteration and Restoration in the 21st Century

Large rivers of the midcontinent of North America have been dammed, channelized, stabilized, and leveed to maximize their socio-economic contributions.  However, with emerging understanding of the ecological costs of alteration, restoration is flourishing in the early 21^st century.  Restoration of large rivers requires understanding several key concepts.  1) Large rivers can’t be restored to a pristine condition.  Reference conditions are obscure for dynamic systems and lack relevance in rivers that will continue to be highly engineered.  2) Large rivers lack replicates; each is unique in its biophysical capacity, alteration history, and socio-economic constraints.  History and geography set restoration context. 3) Physical habitat has been highly altered on these rivers; direct action on physical habitat is the most common approach but sufficiency is often unclear.  4)  Whereas flow regime can be considered the master variable in smaller rivers, it is not on large rivers where flow regime and channel form are decoupled.   Large-river restoration science in the 21^st Century is greatly enhanced by hydroacoustics and hydrodynamic modeling tools that support detailed analyses of flow and form, and by telemetry tools that place large fish into habitat context.  Challenges remain in linking physical habitat to ecological, population, and community dynamics.