Dams And Rivers: Scientists Take A New Look Downstream

December 18, 1996

Dams provide many benefits -- reducing flood hazards, providing reliable water supplies, producing hydroelectric power, and providing places for flatwater boating -- but with those benefits come environmental consequences -- eroding river banks, changes in waterfowl habitat, concerns for safe recreational use, and the loss of river sand bars, according to a new report by the U.S. Geological Survey.

"Dams and Rivers: Primer on the Downstream Effects of Dams," the new USGS report, outlines the role of science in restoring or otherwise altering these downstream effects.

"The USGS has spent more than 100 years monitoring the nation's rivers and streams and providing the hydrologic data needed to design the nation's infrastructure of dams,"said Robert M. Hirsch, USGS chief hydrologist, Reston, Va. "Today, the era of dam building in the United States is largely over and the USGS has turned some of its monitoring and research focus towards providing dam managers and resource planners with the kind of in-depth information they need to reduce some of the more harmful downstream effects of dams on river environments."

The USGS report looks at dams and rivers in seven selected areas of the country -- the Upper Salt River in central Arizona; the Snake River in Idaho, Oregon and Washington; the Rio Grande in New Mexico and Texas; the Chattahoochee River in Georgia; the Platte River in Wyoming, Colorado and Nebraska; the Green River in Utah; and the Colorado River in Arizona -- to focus on specific downstream effects of dams and the management issues related to their operation.

The 94-page, richly illustrated USGS report has a special section on the role of science in the management of dams to minimize downstream impacts. For example, research provides hard data on environmental changes that might occur if water releases are altered. By using computer modeling, management plans can be developed that best balance needs of users with concerns for the environment. Monitoring and long-term data sets of streamflow trends can help in the development of better predictions of the movement of water and sediment below dams.

"The downstream effects of dams are a subject of much public controversy and debate," said Robert Hirsch, USGS chief hydrologist. "In asking our scientists to produce this report, we wanted to emphasize the important role that science has in helping society to make informed decisions about the management of the nation's dams and river systems.

"Scientists are increasingly being called upon to suggest dam operation regimes that will minimize a particular negative impact to the downstream environment," Hirsch said. "Scientific insights gained from hydrology, however, must be integrated with recommendations from the fields of biology, economics and engineering. We must look more and more to interdisciplinary scientific approaches to help those who manage the nation's water and other natural resources.

"Any scientific recommendation that alters a dam's operation must be evaluated in the context of the people whose lives will be most directly affected -- people whose interests may be as disparate as the river runner who wants steady in-stream flow and the farmer who wants to irrigate crops with low-salinity water," Hirsch said. "To better balance those issues in the management debate, we must be able to provide the critical science foundation upon which sound and effective decisions can be made."

Highlights from the featured rivers and the issues they represent:

The Upper Salt River, a natural stream in central Arizona, has all of the characteristics of a healthy, unregulated river. Flow and sediment transport vary widely from one season to another and from year to year. Occasional high-magnitude floods move cobbles and boulders into bars, which form rapids that attract recreationists. Native riparian (along the river bank) vegetation forms a dense band above flood level and non-native vegetation is sparse. The river is always in the process of adjusting its channel to the equilibrium that exists between erosion and sediment deposition. The Salt River offers a standard against which to compare regulated rivers.

The Snake River in Oregon, Idaho and Washington is one of the most extensively dammed rivers in the West. The Hells Canyon Complex of dams on the Snake River in Oregon, Idaho and Washington have severely altered the normal dynamics of the river's flow. The dams have limited reservoir storage and limited value for flood control and are used to provide hydroelectric power at times of peak demand through a grid system that provides electricity throughout the West. Dams on the Snake block historical salmon spawning runs. Frequent high-flow releases have caused depletion of sand in the river channel downstream from the dams.

The Rio Grande is, in a sense, one river cut and transformed into two -- a vigorous and unregulated snow-fed river in northern New Mexico and a highly regulated river with substantial flow where it reaches the Gulf of Mexico. The dam was designed to retain all flow on the Rio Grande, releasing water only for irrigation purposes. As a result of this restricted flow, sediment loads, which normally are resuspended and transported downstream during high flows, have accumulated on the river bed and the channel has been invaded by tamarisk (a non-native, salt-tolerant bushy tree) for hundreds of miles below the dam. The channel has diminished in size since the dam was constructed and can no longer contain floods as large as it once could. As a consequence, relatively minor floods have caused significant damage to riverfront properties and structures downstream from the dam.

The Chattahoochee River in Georgia is managed with recreation as a high priority, including boating and prize trout fishing. Lake Lanier above Buford Dam is the most heavily visited, federally managed lake in the country. Buford Dam is also used to meet peak-power demands of the Southeast hydroelectric power grid. The consequences of these patterns of frequent water release affect the safety of recreational users and have resulted in erosion of the river banks downstream.

The channel of the Platte River in Wyoming, Colorado and Nebraska has been narrowed to as little as 15 percent of its former width as a result of in-channel sediment accumulation in some stretches, caused by the placement of dams upstream. The numerous dams and reservoirs that provide flow regulation for irrigation have also depleted much of the Platte River's volume and significantly reduced the magnitude of spring floods. The riparian habitat, depended on by various species of cranes and other waterfowl, has also been severely restricted. Balancing the need for irrigation water for farmers upstream and for restoring wildfowl habitat downstream is one of the management challenges being faced. Adaptive management techniques have been suggested that would allow for moderate releases that could submerge sandbars that would otherwise host germination of unwanted vegetation. Under such a management strategy, planned releases could also open and maintain a channel adequate for use by waterfowl.

The Green River in Utah, Wyoming and Colorado has had its channel profoundly altered and its water temperature affected by the construction and operation of Flaming Gorge Dam. Native fish were sufficiently threatened by these changes that the U.S. Fish and Wildlife Service invoked the Endangered Species Act to mandate dam releases deemed least likely to harm native fish. Dam operations have been modified in ways thought to be beneficial to the habitat of native fish. Collaborative work on the operation of Flaming Gorge Dam sets a precedent for a cooperative approach to minimizing the problems that exist below dams.

The Colorado River is the very heart of the Grand Canyon in Arizona. Glen Canyon Dam, which impounds Lake Powell, traps the vast quantity of sediment that once flowed through Grand Canyon. The clear, cold water that is discharged from the dam and the elimination of annual flooding have altered sediment transport and the biological communities along the Colorado River in Grand Canyon National Park. Glen Canyon Dam is authorized for flood control, recreation and hydroelectric power in addition to its water storage and distribution function. To balance these many mandates, studies that provide scientific input into dam operation suggest the benefit of occasional, specifically designed high-flow releases -- beach-building flows. Such a controlled flood was conducted in April 1996 downstream from the dam and was designed to resuspend and transport accumulated sediments at the bottom of the channel. In rebuilding sand bars and beaches along the river, this controlled flood experiment demonstrated that such engineered floods can have a beneficial effect and that dam management strategies can be developed to allow for such periodic events.

Written for a general audience, the USGS report also provides an extensive bibliography of available resources for a more technical investigation on the general topic and the specific river systems.

Single copies of the report, published as USGS Circular 1126, "Dams and Rivers: Primer on the Downstream Effects of Dams," by Michael Collier, Robert H. Webb and John C. Schmidt, are available free of charge from the Branch of Information Services, U.S. Geological Survey, Box 25286, Denver Federal Center, Denver, Colo. 80225.

* * * USGS * * *

(Note to Editors: Review copies of the report for media are available from the USGS Office of Outreach, 119 National Center, Reston, VA 20192; telephone: 703-648-4460.)

US Geological Survey

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