Effects of upstream hydropower operation on riverine particle transport and turbidity in downstream lakes

[ 1] Retention in upstream storage dams results in modified riverine water and particle discharge patterns. Particularly, suspended solids input and intrusion dynamics in downstream lakes are affected by dam operations. In a case study, size-dependent particle budgets for peri-alpine Lake Brienz ( Switzerland), downstream of major hydropower installations, were determined for a recent 8-year period ( 1997-2004) and compared to hypothetical no-dam scenarios based on numerical simulations. For this purpose, current tributary particle loads, as well as lake-internal sedimentation and turbidity dynamics, were assessed with in situ measurements. The analysis shows that hydropower damming drastically diminishes particle fluxes and minimizes ( short-term) peak discharges. Reductions of high-flow events substantially cut the number of deep intrusions increasing particle supply to the lake surface layer. Furthermore, these hydropower operations shift particle inputs from summer to winter. As a consequence, such peri-alpine lakes become more turbid during winter and less turbid during summer, influencing the seasonal light regime and subsequently the dynamics of phytoplankton growth.