The Paria River, which drains the Grand Staircase and Bryce Canyon, is a major source of sand, deposited at the stream's confluence with the Colorado 15 miles downstream from the dam.
"The scientists estimate 1.5 metric tons are in the main stem and that's three times more than last year," Castle said in a phone interview from the dam in Page, Ariz. "We can do a release and spread that sediment up onto the bed and banks and use that to reduce erosion that has occurred to beaches, sandbars and backwaters."
Since it began holding back the river in 1963, the dam has prevented sediments from replenishing sandbars in the Grand Canyon and disrupted river hydrology. Most of the sediments carried by the Colorado are trapped behind the 710-foot-high concrete arch dam that impounds Lake Powell.
Meanwhile, the dam's tamed flow is rarely sufficient to flush exposed sediments that accumulate below the dam. Then-Interior Secretary Ken Salazar last year authorized a 10-year experiment to allow the bureau to release big flows when the conditions are right, like they are now thanks to recent rain storms, according to Utah State University geomorphologist Jack Schmidt.
"This year was a big monsoon season, particularly in the middle of September," said Schmidt, who leads the U.S. Geological Survey's Grand Canyon Monitoring and Research Center in Flagstaff, Ariz. "There is a lot of sand available for redistributions to do some good in the canyon."
The past four high-flow events were separated by several years, but now they can be pulled off annually.
"We need to do multiple floods if we are going to get a cumulative benefit that will last for a significant amount of time. Many sandbars that are built are subsequently eroded. One [high-flow event] isn't enough," Schmidt said. "We need to see the details of where the bar-building occurs because each flood has a concentrated part of the canyon where it deposits sand. We want to understand the spacial distribution where deposition occurs and understand the benefits of that."
Starting at 1 p.m. Monday, the bureau began ramping up dam releases from 8,000 to 34,100 cubic feet per second (cfs), the highest flow that can currently be released safely. This volume is less than last year's peak flow of 42,000 cfs because two of the dam's six power-generating turbines are out of commission.
By the time releases return to normal on Saturday, Lake Powell's level will have dropped by 2.8 feet and revenues, associated with power that won't be generated, will be reduced. That loss cannot be calculated for several months, but last year's release cost $1.2 million in lost power, according to Castle.
Officials believe such costs will be more than offset by benefits to river resources downstream and by adding to scientists' understanding of the river and the life it supports. But this year's high flows are being released at a time of prolonged drought, when Glen Canyon's annual release has been cut for the first time from its usual 8.23 million acre feet.
"We can do these releases consistently while managing the river for drought because the protocols require us to operate within the Law of the River [enshrined in the multi-state compact apportioning the river's flow] and meet obligations for water delivery and [to] generate power," Castle said.
The Colorado's lower basin states will still get their share of the river, but the timing of water deliveries will be changed to account for the deluge that's about to hit Lake Mead downstream.