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

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.