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They will be slowing the flow at Glen Canyon Dam this week, the latest installment in a series of attempts to analyze and ultimately revive the habitat for endangered native fish species in the lower Colorado River.
Beginning Thursday and continuing through Saturday, the Bureau of Reclamation - which operates the dam - will start reducing Glen Canyon's high-low water fluctuations to roughly half of what they have been this month. The lower flows will continue for the following three weeks, then bureau officials will smooth the releases to a steady level before resuming the reduced fluctuations in October.
The flow adjustments are all part of a larger scheme by federal officials to restore the Colorado River to something closer to its natural habitat in the Grand Canyon, with the goal of reviving fish species such as the humpback chub. This week's flow test follows a Glen Canyon release last November in which flows were substantially increased in a bid to wash sediments down the river to create the sandbars and backwaters that young native fish need to survive.
This time around, bureau officials and scientists will be assessing water temperature changes and the food sources that come downstream in the course of the river fluctuations.
Water flowing naturally downstream warms during the summer, but the water released from Glen Canyon Dam is typically much colder because it comes from the deep recesses of Lake Powell. The new twist, according Dennis Kubly, manager of the bureau's adaptive management plan, is that because of the drought and the reservoir's depleted supply, the water released from Glen Canyon is currently warmer than usual - providing bureau analysts with an opportunity.
"There is strong evidence that these fish will grow faster in warmer water than colder water," Kubly said.
"They'll be larger at the end of the year; they'll have a better chance of survival than the smaller fish."
But as has been the case with the bureau's other flow test experiments, this latest release is drawing flak from some environmental groups, who argue that the experiments have been a failure.
"What they're trying to do is artificially create a natural river habitat with the dam, but that can't be done," said John Weisheit, conservation director of the Moab-based Living Rivers organization. "That 700-foot dam is trapping all the sediment in Lake Powell, and along with it all the nutrition - carbon, trees, driftwood - which creates food base for these fish."
Weisheit says the humpback chub has re-established itself in the Little Colorado River, which feeds into the Colorado. What the bureau is attempting to do, he says, is create a second stronghold - which is a worthy goal, in and of itself.
"But it's not working," he added, "because the sediment continues to be pushed downstream because of the normal fluctuating flows from power generation."
Bureau manager Kubly says it is too early to begin making those kinds of assessments.
"People want to reach conclusions quickly," he said. "But sometimes they do it before the evidence comes out in the science."
Beginning Thursday, Glen Canyon Dam's August flows, which have ranged from 10,000 to 18,000 cubic feet per second (cfs), will be reduced to 6,500 to 9,000 cfs. Those daily fluctuations will be adjusted to a steady 8,000 cfs beginning Sept. 21, with the reduced high-low flows resuming from Oct. 8 through the end of the month.
Normal dam operations will resume on Nov. 1.