The South Santiam River in southeastern Linn County dwindles to a knee-deep channel of water by late August, while the Willamette River maintains a steady and abundant flow through the summer.
In 50 to 100 years, however, warmer climate temperatures likely will have little effect on the summer flows of rivers like the South Santiam, but will impact larger, slower-draining systems such as the Willamette, according to a recent study co-authored by researchers from Oregon State University and the U.S. Forest Service.
The results of the study, led by Mohammad Safeeq, an OSU post-doctoral researcher, were published in the journal Hydrological Processes. OSU released information about the study Monday.
Due to the terrain of the upper Cascades, melted snow and rain seep into the porous underlying rock and begin a slow journey toward river systems including the Willamette, McKenzie, Deschutes, Klamath and Rogue. The slow, continuous flow supplies water to the rivers throughout the summer.
“It’s like a big sponge,” said study co-author Gordon Grant, a hydrologist with the Pacific Northwest Research Station of the Forest Service. “The water comes in (during) the wintertime either as rain or snow and slowly comes out, even through the summer.”
Climate change will lead to the mountain snows melting sooner in the season, Grant said, which will cause the seemingly abundant supply of water to these rivers to run out sooner.
“In slow-draining landscapes, the highest point of the river is going to occur earlier in the year,” Grant said. “It keeps diminishing over the entire period before the fall rains come.”
In contrast, in areas with steep slopes and relatively impermeable rocks, such as the Coast Range or older Cascades, rain and melted snow rapidly run off the land, resulting in high flows in the winter and very little water left in the summer.
Climate change will have little effect on their summer flows because they rely less on snow melt and they already reach their low point in the summer, Grant said.
“It may hit its low point sooner but it (the water) will not get any lower than it already is,” Grant said. “Whereas the slow drainers will continue to fall because they have water to give.”
In their study, the researchers explored daily stream flow data from 81 watersheds across the western United States during the years 1950 to 2010, to explore the drainage efficiency and snowpack dynamics of the systems.
“We looked at 61 years of records, and it looks like Cascade streams today have an average summer flow that is about two centimeters lower — or about a 36 percent decline — over historical averages,” Safeeq said.
They also looked at rain-driven systems and discovered these, too, have experienced declining stream flow in late fall and winter.
Safeeq, a post-doctoral researcher in OSU’s College of Earth, Ocean, and Atmospheric Sciences, noted that no one previously had looked at the magnitude of retrospective stream flow change in different river basins “through the lens of their hydrogeologic differences.”
“They act differently and in ways many scientists may not have predicted,” Safeeq said. “The bottom line is that slow-draining, snow-driven river systems may appear to be less affected by climate change, but they are in fact most sensitive to change.”
Grant noted that the study shows that “we have to look beyond just knowing where snow will turn to rain in the future to predict stream flows. The geology of the landscape and its effect on how fast water moves is equally important.”
Grant said the research will help give a sense of where water is likely to be abundant and scarce in the future, but he hopes that resource managers don’t panic over the analyses.
“I think it helps us begin to look down the road a bit and begin to anticipate where we could potentially see water shortages in the future,” he said, “and during what season and under what circumstances are we likely to see water scarcity.”