The simultaneous growth in climate‐driven alterations of the hydrologic cycle and global freshwater demand threatens the security of anthropogenic and ecologic uses of streamflows. However, the impact of damming on the response of river regimes to long‐term climate variability has not been fully disclosed yet. Here, this issue is assessed by investigating temporal patterns in the occurrence probability of different flow ranges upstream and downstream of a selection of dams in the Central‐Eastern United States. We found that long‐term fluctuations of low flows are propagated unaltered from unregulated to regulated regimes. In the majority of cases, the same applies to the entire spectrum of streamflows, although discharge interannual variability is significantly amplified by large multipurpose structures. Water supply dams instead smooth long‐term streamflow fluctuations, though at the cost of systematically filtering out medium‐to‐high discharges. Accordingly, in Central‐Eastern United States, dams are unable to mitigate the sensitivity of flow regimes to long‐term hydroclimatic fluctuations and, thus, do not support the security of anthropogenic and ecologic uses of regulated streamflows.

<p>The simultaneous growth in climate-driven fluctuations of river flow regimes and global freshwater demand threatens the security of anthropogenic and ecologic uses of streamflows. Dams have long been designed to reconcile the conflict between patterns of human water uses and the temporal variability of flows, and are operated worldwide. In this context, there is a need to understand the combined influence of reservoir operations and climate variability on regulated streamflow regimes, and disclose whether observed hydroclimatic fluctuations can be accommodated by existing reservoirs. Here, these issues are addressed through a quantitative analysis of flow regime alterations by dams as driven by heterogeneous uses and variable regulation capacities (i.e., storage capacity scaled to the average inflow). In particular, the concept of streamflow stability is used to compare inter-annual changes in the occurrence probability of synchronous flows observed upstream and downstream of dams. The selection of structures considered in this study is distributed throughout the entire Central-Eastern United States, so as to span heterogeneous hydroclimatic settings and reservoir functions (i.e., flood control, water supply, hydropower production and multi-purpose). Our results reveal that reservoirs devoted to flood control and those operating for water supply produce distinctive impacts on flow regimes. Flood control does not alter the mean discharge downstream, but decreases long-term discharge variability and, thus, homogenize regional flow dynamics. However, regulation for flood control is unable to mitigate the impact of variable climate drivers on the stability of streamflows and hydroclimatic fluctuations typical of unregulated regimes are transferred unaltered in downstream reaches, or even amplified. Water supply, instead, reduces the mean flow of regulated reaches but increases the long-term streamflow variability, thereby enhancing the regional heterogeneity of flows. In this case, regulation smooths inter-annual changes of flow regimes, though at the cost of systematically filtering out medium-to-high discharges, with negative consequences on stream ecosystems. The observed connection between reservoir functions and the features of downstream flow regime alterations by dams represents a critical step forward for a sustainable management of water resources.</p>