Climate study foresees fewer year-round streams
Some year-round streams could become intermittent as global climate change takes a toll on the Upper Colorado River basin, according to a recent study.
“Modeled intermittency risk for small streams in the Upper Colorado River Basin under climate change,” written by Lindsay V. Reynolds, Patrick B. Shafroth, and LeRoy Poff of Colorado State University and the United States Geological Survey, analyzed data from 115 stream gauges across Western Colorado and Eastern Utah, with a handful of sites in Wyoming and New Mexico. Although they didn’t single out specific streams, the researchers suggested that those with already-weak or erratic flows were at higher risk.
“What we found were that streams that currently have a variability or lower annual flows, those are the streams that are likely to be more threatened under climate change,” Reynolds said.
Streams categorized as “strongly perennial” were generally higher elevation where snow lingers longer than those tagged as “strongly intermittent” or “weakly intermittent.” Perennial streams also trumped their strongly intermittent counterparts in drainage basin size, but weakly intermittent basin size varied considerably, suggesting that a large watershed may not be proof against dry spells.
Overall, the study indicated that most individual climate variables were poor predictors of stream intermittency compared with the Palmer Drought Severity Index, which incorporates numerous factors. It did, however, establish a relationship between minimum flows — which establish whether a stream will run dry — and mean flows — which are more commonly used in climate models.
“Many climate change scenarios for this region involve decreases in mean annual streamflow, late-summer precipitation and late-summer streamflow in the coming decades,” the report summarizes. “Intermittent streams are already common in this region, and it is likely that minimum flows will decrease and some perennial streams will shift to intermittent flow under climate-driven changes in timing and magnitude of precipitation and runoff, combined with increases in temperature.”
That’s an unnerving forecast for a river basin that has repeatedly made American Rivers’ list of Most Endangered Rivers. This year, the Colorado River in the Grand Canyon is the subject of most concern, last year the Upper Colorado came in second, and in 2013 the whole drainage topped the list.
In addition to the environmental cost, a lot of people rely on water from the Colorado — and by extension its tributaries.
“It’s a critical water source for a huge portion of the Western United States, and the region is predicted to get drier in the future under climate change,” Reynolds said.
And demand is only likely to increase.
“You have the climatic variables and you have the variable of increased human use,” observed Jim Pokrandt, communications director for the Colorado River District. “With warmer temperatures, we’ll use more water anyway. That puts more pressure on rivers and streams.”
Pokrandt held up the Crystal River as “the poster child” for the dual impacts of drought and human use. The Crystal made the most endangered list in 2012, before the potential for a dam project was averted.
Although Pokrandt doesn’t want to see any streams dry up, his biggest concern is how low flow impacts will trickle down the watershed.
“The concern is for the whole river system,” he said. “You’ve got to look at the big picture.”
For Reynolds, the challenge is quite the opposite — taking their broad statistical analysis and seeing what changes look like at ground level.
“We’re interested in looking at what stream drying looks like for stream-dependent ecosystems,” Reynolds said. “In the future we’d like to identify streams where that’s already happened.”
The researchers have already touched on the issue as part of an extensive report to the Bureau of Reclamation.
“Our driest category of streams, intermittent low streams, had the lowest total frequency of plants, which indicates low plant cover in general,” they wrote. Moreover, native plants seemed the hardest hit.
Reynolds hopes the data will help inform scientists and water managers about the potential tangible impacts of climate change on the West.
“It might help us focus in on places to target for conservation,” she said.
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