Environmental Flow Scenarios for a Regulated River System: Projecting Catchment‐Wide Ecosystem Benefits and Consequences for Hydroelectric Production

To enable prioritization among measures for ecological restoration, knowing the expected benefits and consequences of implementation is imperative but rarely explicitly quantified. We developed a novel method to prioritize among environmental flow measures to rehabilitate ecosystems in the Ume River catchment in northern Sweden, a river system heavily regulated for hydropower production. Our strategy was to identify measures with minimal impact on hydropower production while providing substantial environmental benefits. Based on field surveys of remaining natural values and potential for ecological rehabilitation, we quantified the projected gain in habitat area of implementing environmental flows for target organism groups, for example, lotic fish species and riparian vegetation, along the whole river length. We quantified the consequences for hydropower production by identifying a set of hydropower operational rules reflecting the constraints added by environmental flows. We then used production optimization software to calculate changes in hydropower production and revenues. Implementing restrictions on zero‐flow events by mandating minimum discharge at all run‐of‐river hydropower stations and allocating 1%–12% of mean annual discharge to bypassed reaches in the entire catchment would result in a 2.1% loss of annual electricity production. Adding flow to fishways would increase the loss to 3.1% per year. With implementation of more natural water‐level fluctuations in run‐of‐river impoundments, the loss increases to 3.8%. These actions would increase the habitat for lotic species like the grayling Thymallus more than threefold and increase the area of riparian vegetation by about 66%. Our method forms a basis for ongoing implementation of nationwide environmental rehabilitation schemes. Plain Language Summary Developing river systems to produce hydropower results in impoverished riverine ecosystems. Legal demands to strike a balance between the provision of renewable electricity and consideration of the value of riverine ecosystems have led to calls for ecological rehabilitation of river systems to conserve their natural values. To achieve this, environmental rehabilitation actions are needed, including quantifying environmental benefits and consequences for hydropower production at the catchment level. We developed a novel methodology for the “cost‐benefit analyses” needed and made an environmental flow assessment of the heavily regulated Ume River syst