Effective dam removal and river channel restoration approaches

Abstract Millions of dams have been built on the planet's rivers and streams to provide societal needs for freshwater supplies, irrigation, hydroelectric development, industrial activities and flood control. The structural and operational components of most dams include features that reflect technological advancements and changing societal priorities over time. Many dams were constructed and operated with no consideration for river ecology; some dams have included eco‐friendly features (e.g. fish passage facilities, instream flow releases); other dams were modified, replaced or removed. Dams have left their mark on the landscape and the biota that depend on rivers for their entire life cycle or for a part of it. Aquatic organisms (e.g. fish) are particularly sensitive to changes in habitat connectivity, hydrograph alterations, morphodynamic modifications, as well as hydrogeologic, thermal or chemical deviations from natural levels. As various societies around the globe, particularly in recent decades, place a higher priority on river ecosystem health and sustainability, effective approaches to deal with existing and new dams are emerging. Restoration of fluvial processes and ecological functions of rivers may take several forms, including dam replacement or removal. Simple removal of dams can result in sedimentation of downstream habitat, and channel adjustment within the former reservoir may be a long term process before a stable pattern, dimension, and profile becomes established. An alternative approach is to use natural channel design techniques to stabilize accumulated sediment and advance recovery towards a geomorphically stable and ecologically diverse stream. Two approaches and case examples are discussed, one removal with no channel restoration, and one removal with channel restoration. Approaches to dam removal and river restoration involve management, engineering and scientific challenges, demand balanced and sensible resolution, and lead to new research opportunities to fill knowledge gaps of the complex interactions between ecological and physical river processes.