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Worldwide, water conflicts are escalating as cities, industries, agriculture and energy producers compete for limited freshwater supplies. At the same time, there is a growing awareness of the need to maintain adequate freshwater flows in rivers, lakes, floodplains, aquifers and estuaries to sustain biodiversity and the many benefits derived from the healthy, functioning ecosystems upon which local communities and economies depend. Efficient, integrated water resource management systems help governments provide for growing human populations while protecting and restoring healthy freshwater ecosystems.

The integration of ecological considerations into water management has been hampered by the difficulty, cost and time required for determining environmental flows – the quantity, timing and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and well-being that depend on these ecosystems. Environmental flow assessments scientifically evaluate tradeoffs between alteration of natural water flow patterns by humans and consequent changes in ecological health.

Despite the hundreds of methods available for assessing environmental flow needs, many are not based on sound ecological principles or are simply impractical for application at the broad regional scales at which governments manage water resources. Simple ""rules of thumb"" lack scientific credibility, while more sophisticated, data-intensive methods are too expensive and time consuming to apply to every freshwater system within a large jurisdiction.

The Ecological Limits of Hydrologic Alteration (ELOHA) is a framework offering a flexible, scientifically defensible compromise for broadly assessing environmental flow needs when in-depth studies cannot be performed for all rivers in a region. ELOHA builds upon the wealth of knowledge gained from decades of river-specific studies, and applies that knowledge to geographic areas as large as a state, province, nation, or large river basin.