Restoration of former channels: What do we mean?

Background/Question/Methods Oxbow lakes, sloughs and side channels and other off-channel water bodies are created by geomorphic processes such as channel migration, chute-cutoff, and avulsion, and typically go through an evolutionary sequence in which sedimentation gradually converts them from aquatic to terrestrial environments. Many such former channels have lost ecological function through human action, such as artificial cutoffs to shorten navigation routes, levees that cut off hydrologic connection, filling for agriculture, pollution from runoff from adjacent lands, or loss of connectivity with groundwater due to sedimentation. How successful have been attempts to ‘restore' former channel environments? The dynamic nature of these geomorphic features (and the habitats they support) implies that restoration proposals should be keyed directly to the functions desired, and that restoration actions do not attempt to unrealistically halt evolutionary processes. We reviewed case studies of former restoration projects (completed or proposed) on the Missouri, Apalachicola, and Sacramento Rivers in the U.S., and the Ain and Rhone Rivers in France to document performance of various former channel restoration approaches in diverse environments. Results/Conclusions Many former channel restoration projects have been efforts to recreate valued habitats not maintained by fluvial processes, and thus have not proved self-sustaining. Projects to reconnect former channels to the main channel in the Missouri and Rhone Rivers have had various levels of success, depending on local hydraulics and sedimentation. Numerous projects to restore habitats at the mouths of tributaries and other side channels of the Apalachicola River have refilled with sediment within months or years, and re-routing the river into an artificially cut-off meander bend is now proposed as a more sustainable approach. Incision of the mainstem Ain River left side channels too high to be flooded, so restoration involved excavating the bed of a side channel so it connected annually. The sustainability of such restoration actions should be evaluated in the context of the geomorphic processes affecting the evolution of former channels and the management constraints within the river systems.