Thursday, Jun. 03, 2010
Trees will tell the story, says Utah State University doctoral student Rebecca Manners, of dramatic changes over the past century in the nearly 245,000-square mile Colorado River Basin. Manners was awarded a prestigious National Science Foundation grant to study the sediment, water and climate factors that have allowed tamarisk to drastically alter stretches of the Green and Yampa Rivers in northwestern Colorado and the Colorado River in the Grand Canyon.
The recipient of a 2010 National Science Foundation Doctoral Dissertation Improvement Grant, Manners received word May 19 that her proposal was ranked among the top two of 109 submissions by the foundation’s Geography and Spatial Sciences Program advisory panel.
“Rebecca’s work is important to the research and river management communities,” says Jack Schmidt, professor in USU’s Department of Watershed Sciences and Manners’ faculty advisor. “Human society depends on these waters. The Colorado River Basin is also valued for its natural ecosystem and its great recreational opportunities.”
As demands for both water and natural river ecosystems increase, Schmidt says, research such as Manners’ will help society identify tradeoffs that Utahns, and other westerners, will have to make.
Manners conducts research at two study sites within Dinosaur National Monument. One site is on the Green River in Whirlpool Canyon, just west of the river’s confluence with the Yampa River. The other is further east on the Yampa in Yampa Canyon. Using tamarisk trees and stratigraphy data collected from these areas, Manners hopes to shed light on varied factors that are increasingly narrowing river channels.
“Less flow and increased vegetation have altered both rivers, reducing backwater habitats on which endemic fishes rely for survival,” Manners says. “The rivers are becoming less complex in terms of biodiversity, which doesn’t bode well for native plant and animal species.”
The Yampa is a relatively natural river with no major diversions, she says. “It’s the largest unregulated tributary to the Colorado River.”
In contrast, her study site on the Green River, which lies downstream of Flaming Gorge Dam and is affected by the dam’s operations, reflects the impact of nearly a decade of decreased flood flows and sediment supply. Aerial photos taken during the 1930s, juxtaposed with modern-day images, reveal marked narrowing of river channels. Comparison of the Green and Yampa sites allows Manners to assess the impact of diversions, 20th century climate change resulting in less runoff and smaller flows, as well as the spread of the opportunistic tamarisk, on the rivers’ passages.
Even without diversions, the Yampa is also undergoing significant change — much of it attributed to tamarisk, a shrubby import from Asia that has spread throughout the Colorado River Basin and is crowding out native willow and cottonwood trees. Brought by European settlers in the 19th century to reduce erosion, stands of the persistent tree suck up moisture with a surprising voracity that, some contend, rivals the thirst of Colorado River-dependent Las Vegas.
Manners suspects that tamarisk growth is creating a feedback cycle that accelerates channel narrowing, with dense vegetation inhibiting sediment transport, thus fostering an environment even more inviting to further tamarisk growth.
To conduct her study, Manners is employing a variety of high-tech and low-tech tools to measure tamarisk spread and to reconstruct how depositional surfaces formed along the rivers. During the past summer and fall, brawn, as much as brains, has powered her research. Within the national monument, only hand tools are permitted. Rather than backhoes and chainsaws, Manners and her team have used shovels to dig trenches and handsaws to remove trees. Even if power tools were permitted, transporting the equipment into the remote sandstone canyon study sites, accessible only on foot or by raft, would have been cost-prohibitive and probably impossible.
“We’ve dug six trenches — ranging in depth from five to 10 feet and as much as 90 yards long,” she says. “It’s not easy to dig in the desert in July. But I was able to recruit undergrads, fellow grad students, research technicians and personnel from the National Park Service to help. It was quite a heroic effort.”
The reward, she says, was the opportunity to float and hike into strikingly beautiful and untouched areas of the monument.
Study of strata revealed by the trenches and of tamarisk logs — floated downriver for analysis at a U.S. Geological Survey field lab — will contribute to Manners’ study.
She returns to the sites this summer to collect more data she’ll use to construct two-dimensional hydraulic models to test her theories. With WATS faculty member Joe Wheaton, she’ll scan the areas with ground-based LiDAR, a high-definition laser scanning tool used to map geologic surfaces. She’ll also survey the area by air, with assistance from USU civil engineering professor Christopher Neale, capturing multi-spectral images to determine the density of varied types of vegetation, including tamarisk.
“With the aerial images, I’ll conduct a ground truth survey, comparing what we captured from the air with tamarisk stands on the ground,” Manners says.
Study of the two rivers is timely, as each is experiencing development pressure. Waters from the Green supply burgeoning populations in the Southwest; the Yampa is eyed as thirsty communities of Colorado’s Front Range grow. Interest in the area’s potential for oil shale development is also increasing.
“We’re exploring big questions with important consequences,” Manners says. “It’s a privilege to be working in one of the world’s last special places.”
Writer: Mary-Ann Muffoletto, 435-797-3517, firstname.lastname@example.org