Healing Waters: Flooding Rivers to Repent for the Damage Done by Dams
Adler, Tina, Science News
Flooding rivers to repent for the damage done by dams
If Noah had been hanging around the Grand Canyon at the end of March, he'd probably have thought he was having one distorted flashback. He would have seen Department of the Interior Secretary Bruce Babbitt push a button controlling the Glen Canyon Dam and let loose a huge flood of the Colorado River below. As in the Bible story, much preparation preceded the deluge. More than a hundred investigators who had received word of the coming torrent moved some endangered animals to higher ground and set up about 30 projects to study the effects of the flooding.
Babbitt and his crew didn't plan this flood to punish anyone. Instead, they wanted to restore the river to at least a shadow of its former self, before the dam was constructed in 1963. They hoped to improve certain features of the river, such as sandbars, that benefit both native fish species and human campers.
This marked the first time that dam managers have used a large flood to renew the health of a river ecosystem, they say.
The flood began gradually, then flowed at 45,000 cubic feet per second (cfs) for a week. A flood that big hadn't hit the Grand Canyon since a natural deluge in the mid-1980s. Before the dam was built, floods averaging 125,000 cfs occurred annually. In recent years, 12,000 cfs of water would normally travel over the dam during late March.
Huge water flows alter a river's ecology by scouring out backwater lagoons, washing away the banks' vegetation, and moving sand from the bottom toward the shore to create sandbars. New lagoons form behind these sandbars. Ecologists liken the services that floods provide rivers to the benefits that fires offer forests.
Today, most big rivers in the United States are controlled by dams, which floods rarely overpower. As part of its recent efforts to make up for ecosystem changes that have resulted from the lack of flooding, the Interior Department's Bureau of Reclamation decided to go along with scientists' requests to run a trial flood in the Grand Canyon.
Final reports from the researchers who monitored the effects of this $1.5 million flood are due out at the end of the year. A couple of weeks after the event, Interior Department staff were describing it as a resounding success, but scientists who collected data on the river this summer are providing mixed reviews.
High on the project organizers' list of goals for this torrent was creating better conditions for the beleaguered native fish. Only five native species remain of the eight that graced this stretch of the Colorado River before the dam began operation, explains ecologist Richard A. Valdez of Bio/West, an environmental consulting firm in Logan, Utah. Of those five, the humpback chub and razorback sucker are endangered.
Before construction of the dam, the Colorado River ran hot and cold through the Grand Canyon, reaching highs of almost 90#161#F in the summer and dropping to almost freezing in the winter. Now, 45#161#F water from the bottom of Lake Powell gets pumped into the river at the dam, so the temperature rarely exceeds 60#161#F.
The native fish, however, need a warmer environment for spawning and for their young to develop. They still reproduce in tributaries, which are warmer than the river, but there they must endure cramped quarters and an occasional flash flood, Valdez explains. Moreover, the young often die of cold when they leave the tributaries to enter the Colorado. Since the dam began operation, the fish have had fewer nurseries-the warm lagoons where the young mature before competing with the adults in the river.
Without flooding, few new lagoons had developed, and established ones had become overgrown with vegetation.
This year's artificial flood created at least 55 new sandbars, the Interior Department announced at the end of May. More than half of the existing sandbars grew bigger, and only 10 percent lost sediment. …