But under rules implemented in 2012, the high-flow events happen annually as long as conditions warrant.
"So far, so good," said Jack Schmidt, a Utah State University geologist who has led monitoring efforts in the past. "The short-term benefit in having floods in the fall appears to be good for ecosystems, and every time they build sandbars in most places in Grand Canyon."
The world's deepest canyon is hemmed by the nation's two largest reservoirs.
On the upstream side, Lake Powell is plugged by the 722-foot-high Glen Canyon Dam, which keeps both sediments and floodwaters from entering the canyon. About 95 percent of the Colorado's sediments are captured behind the dam, where they will someday fill Lake Powell and stall the towering edifice's turbines.
For now, sediments can enter the Colorado from downstream tributaries, but floods are necessary to push the silt downstream and deposit it on the river's banks.
Reclamation engineers monitor the Paria River, a Colorado tributary 15 miles below the dam, for sediment "slugs." When sediments reach a certain volume, they schedule a flood the following fall or spring.
"You don't do the high flow unless you have a big 'slug' of sediment," said Glen Knowles, Reclamation's chief of adaptive management. "It's an erosive desert stream. During the monsoon season we can get some extreme events, with 1 million metric tons in sediments."
Reclamation guidelines implemented in 2012 require high-flow events in the fall or spring if 200,000 metric tons of sand pile up at the mouth of the Paria.
Over five-day periods in each of the last three Novembers, officials opened flood gates to allow 100,000 acre feet of water to rush past Glen Ganyon's turbines in horizontal jets that could fill an Olympic-sized pool every 2.5 seconds.
"Coincidentally, nature gave us three good monsoon seasons that gave us three good Paria flood years back-to-back-to-back," said Schmidt, the former chief of USGS's Grand Canyon Monitoring and Research Center in Flagstaff, Ariz. "If it happens this fall, the flood will happen again in 2015."
USGS scientists have rigged remote time-lapse cameras along the canyon's 200-mile reach, training their lenses on 43 sandbar locations to document how each responds to the floods and changes over time. The data are shedding new light on how sandbars form, according to project leader Paul Grams, one of Schmidt's former students at USU.
"We are seeing floods building sandbars in different places," Grams said.
The 2012 flow hit the maximum 45,000 cfs, but the next two were less intense — about 36,000 cfs — because of a multi-year turbine-replacement program at the dam.
The smaller floods still created sandbars, but in places where the higher floods didn't.