Each fall, Northern Arizona University geologist Joe Hazel and a team of about 20 others pack several hundred pounds of expensive survey equipment into handful of rafts and set off down the Colorado River.
For 25 years, the team has been rafting 225 miles through the Grand Canyon to survey 45 sandbars, measuring their size and volume to track how the riverside beaches are responding to the operations of one of the West’s biggest dams.
For the past two decades, scientists have been particularly interested in how the sandbars along the river, relied upon by campers and wildlife alike, are responding to experimental high flow releases from Glen Canyon Dam. These controlled floods are meant to benefit ecosystems downstream, including the sandbars, which have historically been a defining feature of the river’s banks.
Now, three years after the Bureau of Reclamation made these “high-flow experiment” releases an established protocol, a group of current and former U.S. Geological Survey scientists have concluded that the tactic is indeed showing promise as a way to reverse decades of sandbar declines.
“Three years into the “high-flow experiment” protocol, the releases appear to be achieving the desired effect,” the scientists wrote last week in an article on the American Geophysical Union’s Earth and space science website.
And as evidence for those conclusions, the USGS is pointing in large part to the data produced by Hazel and fellow NAU research associate Matt Kaplinski.
It was around 1990 when scientists first started discussing the concept of releasing larger amounts of water from Glen Canyon Dam as a way to rebuild sandbars along the Colorado River, said Ted Melis, deputy director of the USGS’s Southwest Biological Science Center, which oversees the Grand Canyon Monitoring and Research Center.
With 90 percent of the sediment that historically flowed down the Colorado trapped behind Glen Canyon Dam, the floods are a way to make the best use of the sand flowing into the river from tributaries, especially the Paria River, Melis said. The rush of water released from the dam acts to sweep up sand that has settled onto the riverbed and elevates it onto the banks.
The Bureau of Reclamation conducted the first controlled flood in 1996, followed by another in 2004 and another in 2008. Each time, scientists and river managers studied the effects and tweaked the process, learning more and more about timing the floods to happen after large sediment influxes from the Paria, Melis said.
All those lessons were wrapped into the formal protocol for the floods finalized by the Bureau of Reclamation in 2012 that outlines a framework for how the controlled floods should occur through 2020. Controlled releases are also being considered as a longer-term strategy under a still-in-progress environmental impact statement that will guide dam management for the next 20 years.
Signs in the sand
The USGS article focused its conclusions on the effects of three controlled releases that have happened since the Bureau of Reclamation’s 2012 high-flow experiment protocol was issued.
Though they struck an optimistic tone, the authors and other researchers said calling the controlled releases an outright success for sandbars would be painting the data with too broad of a brush. Time-lapse images from cameras installed along the canyon showed at least half of the monitored sandbars increased in size following each controlled flood from 2012 through 2014, while Hazel’s year-over-year monitoring shows sandbars in the Grand Canyon have generally grown in volume since 1990. But the sandbars in Marble Canyon, in the upper section of the Grand Canyon, haven’t experienced the same growth and have actually declined in volume since 1990, Hazels data shows. The data points also bounce around quite a bit, defying smooth trend lines.
“Basically we know the floods work but there's a lot of variability. Some sites are going to be losers some are gainers and we still don't understand why,” said Hazel, who described himself as cautiously optimistic that the high flow strategy is working.
“It’s promising, but I'm not ready to sign on the dotted line,” he said.
Roger Clark with the Grand Canyon Trust also emphasized that the report contains encouraging news.
"This is definitely a positive step forward in helping out some of those specific resources that previous dam operations have adversely affected," said Clark, who is director of the nonprofit's Grand Canyon program.
Limits to success
The USGS article was careful to emphasize that just because the high-flow experiment appears to be working doesn’t mean that trajectory will continue. Over the last three years, releases from Glen Canyon Dam have dropped to some of their lowest levels since it was constructed, which means less water was rushing down the canyon and washing away sandbars, Melis said. That could change if the Colorado River basin experiences a few wetter years, he said.
Sand inputs from the Paria River also have been higher than usual — 1.9 million metric tons of sand in 2013 and 1.2 million metric tons of sand in 2014, compared to a median of 600,000 to 700,000 metric tons, Melis said. One million metric tons of sand would fill a football field 44 stories high.
The amount of sand flowing into the main stem from the Paria and other tributaries depends on heavy moisture from winter and summer storms. At this point, though, climate change models can’t reliably predict what seasonal thunderstorm activity will look like in the future, the USGS article said.
“It has been a unique period of time at the beginning of the 10-year (high-flow) experiment where conditions have been as good as they could be to rehabilitate sandbars,” Melis said.
In addition to uncertain conditions, the future growth in sandbars will be limited by how high the river’s flows will go. Under the high-flow experiment protocol, the river’s flows are allowed to reach 45,000 cubic feet per second, which is less than half its historical peak flow of about 90,000 cubic feet per second before the dam went up, Melis said.
That, along with lack of sand, will make it nearly impossible to ever restore sandbars to pre-dam condition, he said.
The effectiveness of the high-flow release strategy also depends on frequency. Sandbar measurements show the beaches tend to erode six to 12 months after the floods, so the controlled floods need to occur regularly in order to sustain the sandy banks.
Even with a pile of uncertainties, Hazel said he is encouraged by the effort by water managers to operate the dam for a range of beneficiaries, including the environment downstream.
“The Grand Canyon is a world heritage site and now it’s being managed to help protect the resource and not just generate electricity and store water,” Hazel said. “It’s cool we can balance those things.”
Emery Cowan can be reached at (928) 556-2250 or email@example.com