Ecological effects of SBT operations on a residual river: Solis SBT case-study

Abstract

Sediment trapping and decreased flow variability are two major alterations caused by dams, generating downstream ecological consequences as well as reservoir problems, such as reduced storage capacity. Sediment Bypass Tunnel (SBT) operation is one technique towards re-establishing sediment connectivity and enhancing flow variability in downstream rivers. In this study, we evaluated the ecological effects of a new SBT in an alpine stream (Albula) in Switzerland over 2 years, including 5 major SBT operations. Hyporheic organic matter processing (sediment respiration), primary production and macroinvertebrate assemblages were analysed along a 4-km stretch of the residual river to better understand the ecological impact among different ecosystem properties. Results showed a clear reduction in organic matter processing, primary production and macroinvertebrate density and richness in response to SBT operations. The main factors dictating the impact of SBT operations were the maximum discharge and cumulative volume of sediment released from the SBT. We found temporal and spatial shifts in macroinvertebrate community composition with tributaries playing a positive role in ecosystem recovery following an SBT operation. Based on these results, SBT operations apparently act as short-term (pulse) disturbances to receiving waters, enhancing sediment connectivity and flow variability. Importantly, however, thresholds on the magnitude and frequency of operations should be considered to prevent catastrophic disturbances detrimental to riverine ecosystems downstream.