The drought that has reduced Lake Powell to half of its full pool capacity also has exposed the 30-mile length of the upper Colorado River delta, and that delta is eroding rapidly into Lake Powell toward Glen Canyon Dam, a concerned geoscientist is reporting this week.
Lake Powell, the uppermost major reservoir on the Colorado River, is effectively a great sediment trap for much of the sediment that used to reach the Sea of Cortez, before the Colorado River was extensively dammed, John C. Dohrenwend said. He is talking about it this week at the 55th annual meeting of the Rocky Mountain Section of the Geological Society of America meeting in Durango, Colo.
Dohrenwend has dramatic pictures of exposed upper Colorado River delta taken in March 2003 from Highway 95, near Hite Marina, Utah, just downstream from the confluence of the Colorado and the Dirty Devil rivers. They are a stunning contrast to views of the same area taken in March 2002, when the reservoir was 40 feet higher. (The photos can be downloaded from the UANews.org at http://ali.opi.arizona.edu/silk/request/Upstream_to_95Bridge.jpg and http://ali.opi.arizona.edu/silk/request/Downstream_from_95Bridge.jpg)
"The Colorado River is one of the most sediment-laden streams in North America," Dohrenwend said. "Deltas on the Colorado River, coming into Lake Powell on the north, and on the San Juan River, coming into Lake Powell on the southeast, are today more than 30 miles long, and advance at an average rate of about a mile a year."
During the past 35 to 40 years, sediment at the confluence of the Colorado and Dirty Devil rivers, just upstream from Hite Marina and the present delta front, has grown to about 185 feet thick. Sediments from the recently exposed delta have been eroding at a very rapid rate, Dohrenwend observed.
"In the past several months, the delta front has advanced almost a mile and a half," he said.
Dohrenwend, formerly with the U.S. Geological Survey, now operates a satellite imaging service out of Teasdale, Utah. He is an adjunct geosciences professor at the University of Arizona. He was struck by the view from the Hite Marina overlook last January, when he stopped there on his way to Tucson.
The important issue, beyond the immediate fate of Hite Marina, is "where does all of this sediment go when it reaches the lake?" Dohrenwend said.
Scientists and engineers realized 60 years ago that when a sediment-laden river enters a reservoir, the river water deposits much of its sediment at the upstream end, but also creates a 'density' current that flows along the bottom of the reservoir, dumping sediments all the way along to the face of the dam.
"There is significant sedimentation at the Glen Canyon dam face even now," Dohrenwend said. Added to that, the Colorado River is a desert river with a volume that fluctuates widely, so the reservoir volumes also fluctuate widely, and that further complicates how sediments are deposited behind the dams, he said.
"In this particular case, since this reservoir is now only half full and," if experts are correct, "will stay that way for at least the next year, then all the sediment that has been deposited at the upstream end, which is water saturated, is ready to move," Dohrenwend said. Therefore, any major river flows coming down the system can be expected to erode the saturated upstream sediments rapidly towards the face of the dam.
"We don't know, but this could significantly shorten the useful life of the dam," he added. If sediments reach the water intakes that run power generators, for example, the dam would no longer be useful for power generation. It might still be useful for water storage, but may become less useful for flood control. As more of the reservoir fills with silt, even a run-of-the-mill flood might overtop the dam, Dohrenwend noted.
"My point is, who is studying this?" Dohrenwend said. "It's a time-sensitive problem that needs to be studied. We need to study it, if only to learn more about how sediment will be deposited in major reservoirs so that we can more accurately forecast the useful life of these reservoirs. Or perhaps we might develop some ideas of what we might do, or what future generations might do, when it's time to decommission these things."