The quantity, timing, and quality of water flows required to sustain freshwater ecosystems and the human livelihoods that depend on them. Ecological flow assessments guide water allocation decisions to balance human and environmental needs.
Ecological flow (also called environmental flow or e-flow) describes the flow regime needed to maintain the ecological health and ecosystem services of rivers, wetlands, and estuaries. The concept recognizes that aquatic ecosystems are adapted to natural flow variability, and that alterations to the magnitude, frequency, duration, timing, and rate of change of flows can degrade ecological conditions. The natural flow paradigm, articulated by Poff et al. (1997), argues that the full range of natural hydrological variability is essential to sustain the full diversity of native species and ecological processes. Methods for determining ecological flows range from simple hydrological rules (e.g., Tennant method, which prescribes a minimum percentage of mean annual flow) to holistic frameworks (ELOHA, DRIFT, BBM) that consider the flow needs of multiple ecosystem components. The Brisbane Declaration (2007, updated 2018) established ecological flows as an international priority for freshwater conservation and sustainable water management. Implementation remains challenging because allocating water for ecosystems often competes with agricultural, municipal, and industrial demands. Over 90% of the world's river systems are now affected by dams, diversions, and extractions that alter natural flow regimes, making ecological flow assessment and implementation a critical conservation need globally.