A tale of two tailwaters: Constraints to effective mitigation with environmental flow restoration

Background/Question/Methods

Many dams which regulate streamflows in the United States are over 50 years old.     Modernization, optimization, rehabilitation, and decommissioning US dams programs are priorities for the existing dam infrastructure.  In many cases the planning for U.S. dams was conducted before Fish and Wildlife Coordination Act required consultation with the Fish and Wildlife Service and state agencies.   We review the historical changes in two tailwaters that have been controlled by dams from over 50 years.    The Cheoah River has been regulated by Santeetlah dam since 1928. Prior to relicensing agreement there were no flow releases from Santeetlah dam and in 2005 seasonally variable peak and baseflow releases were required.  The Smith River was regulated by Philpott Dam, a U.S. Army Corps of Engineers project since 1950.    We studied the physical habitat, flow, temperature, and fish assemblage characteristics over multiple years.    We develop a conceptual model of changes in river morphology following dam construction and flow alteration.

Results/Conclusions

Both tailwaters showed patterns  consistent with the conceptual model.   The Cheoah tailwater had substantial riparian encroachment in channel and on bars; restoration of peak flows has resulted in the break-up of some midchannel bars.  Fish assemblages in these tailwaters are strongly influenced by temperature regimes, channel morphology, and substrate.  Fish species richness and benthic macroinvertebrate richness in both tailwaters is lower than expected    A coldwater release, daily fluctuation in discharge, sediment size, and lack of connectivity were constraints on downstream flora and fauna.  Operational changes to U.S. Corps of Engineers dams are limited to changes that are within their operating guidelines.  For Philpott Dam the mitigation was a change in the rate of increase in flow, which did influence the peak shear stress on the streambed.  The flow restoration at Santeetlah dam resulted in a 2 degree increase in summer water temperature.     Cheoah River fauna was influenced by a warmer water released from the dam.   Only one fish species re-colonized the tailwater after flow restoration and one fish species diminished in abundance.     In situations such as these, temperature and sediment constraints suggest that restoring flow alone will provide only incremental, if any, change in biota.      Programs to restore native flora and fauna in these and similar tailwaters will require more than re-establishing more natural flows.   We must consider the need for more active management, depending on the loss of habitat due to the sediment trap effect and/or temperature change, as well as the viability of source populations to re-colonize the tailwaters.