In the spring of 1996, an extraordinary experiment recreated a flood along the Colorado River. Now researchers have drawn conclusions about the long-term effects of the torrent, which was designed to rebuild sandbars and reveal the effects of flooding on the river's ecosystem. The flood built up some beaches, researchers found, and didn't appear to permanently damage plant or animal communities.
The week-long flood began at the Glen Canyon Dam, which generates hydroelectric power by controlling the flow of water into the lower Colorado River basin from Lake Powell. The dam blocks sand from passing into the river and replenishing sandbars downstream. By smoothing seasonal fluctuations in water flow, the dam has also altered the environment to which native species were adapted and has permitted nonnative species to take hold. Researchers hypothesized that a properly timed flood would redistribute sediment and perhaps change the ecosystems.
In late March and early April 1996, researchers spilled up to 1274 cubic meters of water per second through the dam for 1 week--more than five times the flow before and after the test. Nearly 100 geomorphologists and ecologists stationed themselves along a 385-kilometer stretch of river downstream of the dam, through the Glen, Marble, and Grand canyons, gathering data on water flow and quality, sediment transport, and aquatic and terrestrial ecosystems. In the weeks and months after the flood, researchers studied how plant and animal populations living in and along the river recovered.
Several sets of data have been released before, but the new reports, in the July issue of Ecological Applications, synthesize the results of the floods after 5 years. The flood redistributed sediment into sandbars more quickly than the researchers expected--within the first 48 hours--suggesting that brief, high-intensity floods will do the job, they report. They also found that fish and other aquatic organisms recovered from the flood within about 7 months; in some cases, native species weathered the flood better than invasive species.
Although the experiment provided valuable insights about this stretch of the Colorado River, says Victor Baker, a fluvial morphologist at the University of Arizona in Tucson, controlled floods may have different effects below other dams. Even so, others point out, the torrent shored up beaches that are used by rafters. "Prior to the Glen Canyon release, it wasn't clear whether [we could] replicate natural processes to produce beaches," says then-Secretary of the Interior Bruce Babbitt, who initiated the 1996 flood by opening the gates to the Glen Canyon Dam spillways. The managed flood, he notes, "proved conclusively that we can."