Environmental flow
‘’’Environmental flows’’’ describe the quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s and the human livelihoods and well being that depend on these ecosystems. Through implementation of environmental flows, water managers strive to achieve a flow regime, or pattern, that provides for human uses and maintains the essential processes required to support healthy river ecosystems. Environmental flows do not necessarily require restoring the natural, pristine flow patterns that would occur absent human development, use, and diversion but, instead, are intended to produce a broader set of values and benefits from rivers than from management focused strictly on water supply, energy, recreation, or flood control.

Rivers are parts of integrated systems that include floodplains and riparian corridors. Collectively these systems provide a large suite of benefits. However, the world’s rivers are increasingly being altered through the construction of dams, diversions, and levees. More than half of the world’s large rivers are dammed, a figure that continues to increase. Almost 1,000 dams are planned or under construction in South America and 50 new dams are planned on China’s Yangtze River alone. Dams and other river structures change the downstream flow patterns and consequently affect water quality, temperature, sediment movement and deposition, fish and wildlife, and the livelihoods of people who depend on healthy river ecosystems. Environmental flows seek to maintain these river functions while at the same time providing for traditional offstream benefits.

Evolution of environmental flow concepts and recognition

From the turn of the 20th century through the 1960s, water management in developed nations focused largely on maximizing flood protection, water supplies, and hydropower generation. During the 1970s, the ecological and economic effects of these projects prompted scientists to seek ways to modify dam operations to maintain certain fish species. The initial focus was on determining the minimum flow necessary to preserve an individual species, such as trout, in a river. Environmental flows evolved from this concept of “minimum flows” and, later, “instream flows,” which emphasized the need to keep water within river channels.

By the 1990s, scientists came to realize that the biological and social systems supported by rivers are too complicated to be summarized by a single minimum flow requirement. Since the 1990s, restoring and maintaining more comprehensive environmental flows has gained increasing support, as has the capability of scientists and engineers to define these flows to maintain the full spectrum of riverine species, processes and services. Furthermore, implementation has evolved from dam reoperation to an integration of all aspects of water management, including groundwater and surface water diversions and return flows, as well as land use and storm water management. The science to support regional-scale environmental flow determination and management has likewise advanced.

In a global survey of water specialists undertaken in 2003 to gauge perceptions of environmental flow, 88% of the 272 respondents agreed that the concept is essential for sustainably managing water resources and meeting the long-term needs of people. In 2007, the Brisbane Declaration on Environmental Flows was endorsed by more than 750 practitioners from more than 50 countries. The declaration announced an official pledge to work together to protect and restore the world’s rivers and lakes. By 2010, many countries throughout the world had adopted environmental flow policies, although their implementation remains a challenge.

Flow Regime and Components

Flow regime influences the water quality, energy cycles, biotic interactions, and habitat of rivers. It is possible to describe flow regime in terms of five states or environmental flow components, each of which supports specific ecological functions. The health and integrity of river systems ultimately depend on these components, which may vary seasonally:
  • Extreme low flows occur during drought. Extreme low flows are associated with reduced connectivity and limited species migration. During a period of natural extreme low flows, native species are likely to out-compete exotic species that have not adapted to these very low flows. Maintaining extreme low flows at their natural level can increase the abundance and survival rate of native species, improve habitat during drought, and increase vegetation.
  • Low flows, sometimes called base flows, occur for the majority of the year. Low flows maintain adequate habitat, temperature, dissolved oxygen, and chemistry for aquatic organisms; drinking water for terrestrial animals; and soil moisture for plants. Stable low flows support feeding and spawning activities of fish, offering both recreational and ecological benefits.
  • High flow pulses occur after periods of precipitation and are contained within the natural banks of the river. High flows generally lead to decreased water temperature and increased dissolved oxygen. These events also prevent vegetation from invading river channels and can wash out plants, delivering large amounts of sediment and organic matter downstream in the process. High flows also move and scour gravels for native and recreational fish spawning and suppress non-native fish populations, algae, and beaver dams.
  • Small floods occur every two to ten years. These events enable migration to flood plains, wetlands, and other habitats that act as breeding grounds and provide resources to many species. Small floods also aid the reproduction process of native riparian plants and can decrease the density of non-native species. Increases in native waterfowl, livestock grazing, rice cultivation, and fishery production have also been linked to small floods.
  • Large floods take place infrequently. They can change the path of the river, form new habitat, and move large amounts of sediment and plant matter. Large floods also disperse plant seeds and provide seedlings with prolonged access to soil moisture. Importantly, large floods inundate connected floodplains, providing safe, warm, nutrient-rich nursery areas for juvenile fish.

Each of these flow components, or events, may be quantified in terms of its:
  • Magnitude: the volumetric flow rate or level; for example, 100 cubic meters per second
  • Timing: the time of year during which a flow event occurs; for example, August
  • Duration: how long an event lasts; for example, 3 weeks
  • Frequency: how often the event occurs; for example, every 2-3 years
  • Rate of change: the rate at which flows or levels increase or decrease in magnitude over time; for example, a 0.2 meter-per-day flood recession rate.

Environmental flow prescriptions, or recommendations, are often expressed in these terms.


One effort currently underway to restore environmental flows is the Sustainable Rivers Project, a collaboration between The Nature Conservancy (TNC) and U.S. Army Corps of Engineers (USACE), which is the largest water manager in the United States. Since 2002, TNC and the USACE have been working to define and implement environmental flows by altering the operations of USACE dams in 8 rivers across 12 states. Dam reoperation to release environmental flows, in combination with floodplain restoration, has in some instances increased the water available for hydropower production while reducing flood risk.

Arizona’s Bill Williams River, flowing downstream of Alamo Dam, is one of the rivers featured in the Sustainable Rivers Project. Having discussed modifying dam operations since the early 1990’s, local stakeholders began to work with TNC and USACE in 2005 to identify specific strategies for improving the ecological health and biodiversity of the river basin downstream from the dam. Scientists compiled the best available information and worked together to define environmental flows for the Bill Williams River. While not all of the recommended environmental flow components could be implemented immediately, the USACE has changed its operations of Alamo Dam to incorporate more natural low flows and controlled floods. Ongoing monitoring is capturing resulting ecological responses such as rejuvenation of native willow-cottonwood forest, suppression of invasive and non-native tamarisk, restoration of more natural densities of beaver dams and associated lotic-lentic habitat, changes in aquatic insect populations, and enhanced groundwater recharge. USACE engineers continue to consult with scientists on a regular basis and use the monitoring results to further refine operations of the dam.

Another case in which stakeholders developed environmental flow recommendations is Honduras’ Patuca III Hydropower Project. The Patuca River, the second longest river in Central America, has supported fish populations, nourished crops, and enabled navigation for many indigenous communities, including the Tawahka, Pech, and Miskito Indians, for hundreds of years. To protect the ecological health of the largest undisturbed rainforest north of the Amazon and its inhabitants, TNC and Empresa Nacional de Energia Electrica (ENEE, the agency responsible for the project) agreed to study and determine flows necessary to sustain the health of human and natural communities along the river. Due to very limited available data, innovative approaches were developed for estimating flow needs based on experiences and observations of the local people who depend on this nearly pristine river reach.

Methods, Tools, and Models

More than 200 methods are used worldwide to prescribe river flows needed to maintain healthy rivers. However, very few of these are comprehensive and holistic, accounting for seasonal and inter-annual flow variation needed to support the whole range of ecosystem services that healthy rivers provide. Such comprehensive approaches include DRIFT (Downstream Response to Imposed Flow Transformation), BBM (Building Block Methodology), and the “Savannah Process” for site-specific environmental flow assessment, and ELOHA (Ecological Limits of Hydrologic Alteration) for regional-scale water resource planning and management. The “best” method, or more likely, methods, for a given situation depends on the amount of resources and data available, the most important issues, and the level of certainty required. To facilitate environmental flow prescriptions, a number of computer models and tools have been developed by groups such as the USACE’s Hydrologic Engineering Center to capture flow requirements defined in a workshop setting (e.g., HEC-RPT) or to evaluate the implications of environmental flow implementation (e.g., HEC-ResSim, HEC-RAS, and HEC-EFM).
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