Why Rivers Need to Flow — High and Low — Again

Don’t be surprised if the next time you head down to a river with a fishing pole you pull up a lowly carp instead of a prized native trout. Most rivers no longer flow the way they’re supposed to flow–and that’s changing the mix of fish and other organisms that call them home.

That’s a key finding of the most extensive study ever done of changes in river flows across the United States. Scientists with the U.S. Geological Survey (USGS) examined flow patterns at 2,888 river locations and compared what they found with the flows they’d expect to find under natural conditions. They focused on changes in the magnitude of flows–the highs and lows that are so crucial to giving river creatures the habitats and life-cycle cues they need to survive.

Photograph of the Grand Canyon in by Michael Nichols.



(See more photos of scenic rivers.)

What ecologist Daren Carlisle and his colleagues found is nothing short of a wake-up call to make the restoration of river flows a high priority. Flows were altered in nearly nine in ten river segments, and compared with eight other variables–including water temperature, nitrogen and phosphorus pollution, and the loss of riverside land to farming or urban uses–stream-flow alteration was the primary predictor of a river’s biological integrity.

The study appeared October 25 in the online version of Frontiers in Ecology and the Environment, a journal of the Ecological Society of America.

( Read the paper.)

Every river has a natural rhythm of flow, a pattern of highs and lows that varies with the seasons. Fish, mussels, salamanders, and other life forms have become adapted to these flows over thousands of years. They depend on the habitats these flows create and the cues they provide to take some action, such as to migrate and spawn. In rivers with severely diminished flows, for instance, native trout don’t get the fast-moving water and gravel bottoms they need, so non-native species more adapted to pond-like habitats, such as carp, replace the trout.

Such changes can begin to unravel a whole web of life, which in turn can eliminate crucial services we humans get from healthy rivers and streams–including the purification of our drinking water. For example, many freshwater mussels disperse by hitching a ride with a particular species of fish before dropping off and nestling into a streambed. If that fish disappears from the river, so may the mussels. That’s bad news for water quality as a single mussel can filter up to a gallon of water per hour.

A leading cause of altered river flows is the operation ofdams and reservoirs to generate hydroelectric power, control floods, and supply water for drinking and irrigation. Dams clearly provide important benefits. But it’s time to ask a question we haven’t yet systematically asked: can dams provide the benefits we need while also giving river creatures the crucial flows they need to survive?

In many cases the answer is yes. But we need to start asking the question–river by river, dam by dam. A good deal of life depends on it.


Sandra Postel is director of the Global Water Policy Project and lead water expert for National Geographic’s Freshwater Initiative.  She is the author of several acclaimed books, including the award-winning Last Oasis, a Pew Scholar in Conservation and the Environment, and one of the “Scientific American 50.”

[This post has been reformatted for Water Currents.]

Changing Planet


Meet the Author
Sandra Postel is director of the Global Water Policy Project and author of Replenish: The Virtuous Cycle of Water and Prosperity. From 2009-2015, she served as Freshwater Fellow of the National Geographic Society. Sandra is also co-creator of Change the Course, the national water stewardship initiative awarded the 2017 US Water Prize for restoring billions of gallons of water to depleted rivers and wetlands. The recipient of several honorary degrees, she works to bridge science, policy, and practice to promote innovative ways of securing water to meet both human and ecosystem needs.