Balancing the River Bank
If too much cobble travels downstream, the river’s banks and bed can build up and even change the path of the river; too little, and the banks and bed will erode.
“Nature is pretty good at keeping a balance, but when humans interfere, like in the case of building a dam, it takes a lot of work to try and maintain that balance,” said Erwin.
Erwin, who is pursuing her PhD in USU’s department of watershed sciences, is working with her advisor, Jack Schmidt, to help maintain this balance through a practice called sediment budgeting. Similar to financial accounting, it keeps track of the gravel carried into a water system and the gravel swept away.
“We’ve seen major ecological changes anywhere a dam has been built,” said Erwin. “The Glen Canyon Dam, which created Lake Powell, actually withholds sand and gravel from the downstream riverbanks, reducing the sandbars along the river. Below the dam at Jackson Lake, a natural lake in Grand Teton National Park, we would expect the opposite to occur, and to see the river channel accumulating extra sediment.”
Erwin has seen these types of changes firsthand as she’s studied the Snake River dowstream from Jackson Lake Dam.
“Park Service workers at Teton National Park have already noticed significant changes in the riparian zones of the park, especially at Jackson Lake and along the Snake River,” said Erwin. “For example, seasonal overbank flooding used to create a succession of species from the Snake River—first willows and cottonwoods along the water, then conifers further away. The land has lost much of that middle transitional ground, which is an important habitat for the animals that live there.”
Additionally, said Erwin, the timing of backwater and side channel flows has changed spawning conditions for fish.
“The dams that we build to provide a regular and sustainable water supply for farmers, ranchers, and residents can have the opposite, unsettling effect on the rivers themselves,” said Erwin.
Because of these changes, there are two conflicting interests at work: those who want a predictable, reliable water supply; and those who want to preserve the natural environmental balance. Each season, decisions must be made to regulate the flow of water allowed out of the dam.
“To be sure, this conflict creates some difficult decisions, and the best thing we can do right now is to back up those decisions with really solid data,” says Erwin.
To do this, Erwin and other researchers at USU have collaborated with researchers from the U.S. Geological Survey and Montana State University who study vegetation and fisheries to detect and quantify changes in the Snake River. They analyzed sets of aerial photos to measure the shape of the river and better understand what’s driving changes in sediment flux.
The ideal solution developed by Erwin and her colleagues would be a water release schedule that balances the sediment budget, providing for general water consumption needs while maintaining natural sediment movement.
The research team is developing that formula through sophisticated computer modeling, but the data needed to fill in the gaps don’t come without getting dirty—and wet.
“I spend my wintertime in the lab at Utah State dealing with the hydraulics, mathematics, and statistics parts of my research,” said Erwin. “In the summer, though, I am up at Jackson Lake doing fieldwork and collecting samples.”
Gathering sediment data means getting into the middle of the flow of the Snake River—which Erwin admits is “really large”—and wielding a 200-lb. Toutle River sampler suspended across the water to determine how much cobble, or gravel, is traveling down the river. This indicates the river’s bedload transport rate.
This spring, Erwin and her colleagues will wrap up the project and deliver recommendations to the U.S. Park Service. The report will focus on an optimal release schedule for the Jackson Lake Dam that will provide the least possible disruption to Teton National Park’s fragile environment.
“I think this research teaches me how everything is interconnected and how even small changes in the environment can make a big difference,” said Erwin. “It’s been a real privilege to work and live in such beautiful places while helping to solve some of these problems.”