Minimum environmental flow (or ecological flow) is the quantity, timing, and quality of water flow required to sustain freshwater ecosystems and the services they provide to people.
Minimum environmental flow, also termed ecological flow or environmental flow requirement, describes the regime of water flow needed in a river, wetland, or coastal zone to sustain the ecological structure and function of aquatic and riparian ecosystems and the human livelihoods that depend on them. The concept has evolved from simple minimum flow thresholds to recognition that aquatic ecosystems require variability in flow magnitude, frequency, duration, timing, and rate of change that mimics the natural hydrologic regime. Over 200 methods exist for determining environmental flow requirements, ranging from simple hydrological rules (such as the Tennant method, which recommends 10-30 percent of mean annual flow) to comprehensive holistic approaches like the Building Block Methodology and ELOHA (Ecological Limits of Hydrologic Alteration) framework. Environmental flows are increasingly recognized in water law and policy: many jurisdictions now require environmental flow assessments before approving new water diversions or dam operations. Flow alteration through dams, diversions, and groundwater pumping has degraded aquatic ecosystems worldwide, contributing to declines in freshwater biodiversity, which has decreased by 84 percent since 1970 according to the Living Planet Index. Implementing environmental flows often requires difficult trade-offs with existing water uses, particularly irrigation, and may involve modifying dam operations, retiring water rights, or purchasing water for environmental purposes.