Environmental (or ecological) flows refer to the quality, quantity, timing, and shape of flow regimes that support ecosystem functions, processes and resilience. The natural flow paradigm treats flow as the “master variable” needed to drive natural variation of hydrologic regimes to protect native biodiversity and the evolutionary potential of aquatic and riparian ecosystems.

Ecological flow assessments are concerned with determining the dynamic flow regime required (or the acceptable departure from the original flow regime) to maintain specified, valued features of the ecosystem.

ESSA’s experienced aquatic systems ecologists and hydrologists have rich experience customizing environmental flow assessments according to available data and scientifically defensible/credible degrees of precaution.

Methods for assessing ecological flow needs have evolved, ranging from presumptive flow standards to screening the degree of hydrologic change and risks for specific species and objectives over large spatial areas to site-specific, bottom-up, causally-reasoned functional flow methods applied to specific locations and species (e.g., as in the Ecological Flows Tool). These different methods achieve different levels of scientific rigor, focus on different questions and have widely varying requirements for data but they can all be valuable for advancing understanding of ecological flow needs.