Study provides insight into how dams affect ecology of southwestern Colorado rivers
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The San Miguel and Dolores rivers are both southwestern Colorado waterways that begin high in the San Juan Mountains. Both carve through narrow, red sandstone canyons. Eventually, the two rivers become one when the San Miguel merges into the Dolores and the Dolores with the Colorado River in eastern Utah. But there is one major difference: The Dolores is dammed at McPhee Reservoir near the town of Dolores, while the San Miguel is one of the last free-flowing rivers in the West.
A study of plant traits on these two rivers may provide clues about how riparian habitats will respond to climate change, not just in southwestern Colorado, but across the state and the West. Researchers from Colorado State University recently completed a two-year study on the Dolores and San Miguel rivers, the results of which were presented at the Upper Colorado River Basin Forum in Grand Junction in November.
The study compared two sites on the Dolores (Rico and Bedrock) with two sites on the San Miguel (Placerville and Uravan) by documenting different plant traits at each of the four sites. A “trait” is simply a measureable feature of a plant, such as leaf area, root depth and height. The more diverse these traits are, the higher something called “functional diversity.”
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For both of the upstream sites, Placerville and Rico, functional diversity was higher than it was at the downstream sites, which scientists expected because the downstream sites receive less rainfall. But the Bedrock site, downstream from McPhee Reservoir, had a much lower functional diversity than its sister site of Uravan. This is likely due to the changes in the river’s flow as a result of the dam. With lower functional diversity comes a decreased resistance to invasive species or climate change.
“Dams really do have a huge impact on the downstream ecosystem, and it’s not always talked about,” said Erin Cubley, one of the researchers on the project and a Ph.D. candidate in ecology at Colorado State University. “Dams hold sediments and seeds, they change the flow; they change the processes that are essential in maintaining these ecosystems.”
The dam across the Dolores River that forms McPhee Reservoir is downstream from the small town of Dolores. It forms the fifth biggest reservoir in the state and holds back about 381,000 acre-feet of water. McPhee Reservoir supplies the agricultural irrigation needs of farmers and ranchers south of the Dolores River Basin. The resulting decreased flow below the dam has big impacts on the downstream ecology, Cubley said. A smaller river channel cuts deeper, not wider, and this lowers the groundwater that riparian plants depend upon for survival.
“Riparian species have a big taproot and can access water a few feet down, but if they can’t access groundwater, they die,” Cubley said. “That is what we are seeing at Bedrock.”
By measuring traits and functional diversity instead of specific plant species (which may vary depending on the river and location in the watershed), the study has implications for many of the state’s rivers.
“By using traits, we can look at how similar their traits are and put them into groups and say, ‘OK, we can transfer our findings across rivers that have different species compositions,’” Cubley said.
Another way dams alter the natural flow of the river has to do with spring runoff. Many dams are managed solely with maximum storage capacity in mind, said David M. Merritt, a riparian ecologist with the U.S. Forest Service National Stream and Aquatic Ecology Center. There are easy tweaks water managers can make that will not compromise the power or storage needs of the dam that can also improve the ecological functioning downstream.
For example, one of the most important components of river health is peak flows and flooding with spring runoff. Some species, such as the cottonwood tree, time the release of their seeds to coincide with peak flows. The fluffy white fibers use the river to carry them downstream to hopefully take root in the riverbank. But when dams control the river and don’t allow for this peak flow to happen, it can have a negative effect on cottonwoods, as well as the whole downstream ecosystem.
“If you are a dam operator, it might be easy for you to time a spike that coincides with that historic timing,” Merritt said. “The timing of peak flow is reliant on temperatures with a little variability annually. A dam operator would have tremendous flexibility on when that would occur.”
Effects of climate change
Assuming that the future of the American West will be warmer and drier than it currently is, the research team can model what a future ecosystem might look like: At what point will more drought-resistant plants move in? If you change the flows of a river, then how will the vegetation respond? What would happen if water managers changed dam operations?
“What if climate change is twice as bad or what if it’s not as bad?” Merritt said. “We are scientists who predict change. … We will be able to show predictions of what the vegetation will look like. It’s a model and a technique that can be used on any river anywhere.”