Why Rivers Need to Flow

source : http://newswatch.nationalgeographic.com/2010/12/01/river-flows-usgs-study-postel/

A new study finds that 86 percent of U.S. rivers are impaired.

By Sandra L. Postel
National Geographic Freshwater Fellow

This post is part of a special National Geographic news series and initiative on global water issues.

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. 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 of dams 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 directs the independent Global Water Policy Project and lectures, writes, and consults on international water issues. She is also Freshwater Fellow of the National Geographic Society, and serves as lead water expert for the Society’s freshwater initiative. Sandra is the author of several acclaimed books, including Pillar of Sand: Can the Irrigation Miracle Last? (W.W. Norton, 1999) and Last Oasis (W.W. Norton, 1997), which appears in eight languages and was the basis for a 1997 PBS documentary. She is also co-author, with Brian Richter, of  Rivers for Life. Her essay “Troubled Waters” was selected for Best American Science and Nature Writing. She is a 1995 Pew Scholar in Conservation and the Environment.