Amazingly, it wasn’t until the 1990s that the importance of flow regime to healthy freshwater ecosystems gained recognition. In the 1950s-60s, attention focused on water resource development, leading to the damming and diversion of millions of rivers and streams. In the 1970s, the focus shifted to remediating the consequences of development, particularly water quality degradation. In some places, minimum flows were required to ensure pollution dilution and, later, to keep rivers minimally wetted for targeted fish species. Yet, despite significant water quality improvements (in early industrialized countries), freshwater biodiversity continues to decline precipitously. What’s missing?

Flow is the “master variable” that influences all the other variables – water quality, energy cycling, physical habitat and biotic interactions – that regulate freshwater ecological systems (Karr 1991; Poff et al 1997). Rectifying impacts to one variable, such as water quality, is insufficient to restore a healthy ecosystem if the master variable remains impacted.

The natural hydrologic regime (Poff et al 1997; Arthington et al 1992; Bunn and Arthington ; Sparks 1995) is the characteristic pattern of water quantity, timing and variability in a natural water body. A river’s hydrologic, or flow, regime consists of environmental flow components (Mathews and Richter, 2007; The Nature Conservancy, 2009), each of which can be described in terms of its  magnitude, frequency, duration, timing and rate of change. The integrity of freshwater systems depends on the natural dynamic character of these flow components to regulate ecological processes.

Anthropogenic alteration of the natural hydrologic regime is a major contributor to the global freshwater biodiversity crisis (Bunn and Arthington 2002; Millennium Ecosystem Assessment 2005). Major causes of hydrologic alteration include dams, withdrawals, climate change and land use.

Environmental flows are the quantity and timing of water flows required to maintain the components, functions, processes and resilience of aquatic ecosystems and the goods and services they provide to people. Unlike the natural flow regime, the environmental flow regime allows for some degree of hydrologic alteration. However, environmental flows are intended to mimic the patterns and ecological outcomes of the natural flow regime.

Prescribing environmental flow needs is inherently an interdisciplinary process. For any given water body, The Nature Conservancy and others recommend using a hierarchical framework to identify the holistic environmental flow methodology that will most rapidly lead to implementation and adaptive management of flow provisions.

For more in-depth exploration of environmental flow concepts and practice, The Nature Conservancy offers online and occasional onsite training.

More Recommended Reading
Dyson, M., G. J. J. Bergkamp, and J. Scanlon, eds. 2003. Flow: The Essentials of Environmental Flows. International Union for Conservation of Nature and Natural Resources (IUCN), Gland, Switzerland, and Cambridge, UK.

Postel, S., and B. Richter. 2003. Rivers for Life: Managing Water for People and Nature.  Island Press, Washington, D.C​