How Thick Forests Can Reduce Snowpack

Much of the Willamette Valley’s water arrives in the winter as snow in the forested Cascade mountains. New research shows that the ways we manage those forests can influence how much water flows into rivers during the spring and summer melt.

Anne Nolin, a professor at the University of Nevada, Reno, is part of a group of researchers modeling the Willamette Valley's watershed. They were interested in the ways precipitation travels from where it falls in the Cascade Mountains down to urban centers in the lower Willamette Valley.

Reservoirs behind dams act like a valve between the uplands and the lowlands, Nolin said. Although the reservoirs primarily exist for flood control, they also store water in the winter and spring, and then release it in the summer and fall to ensure flows for salmon and people. But there's an even bigger store of water in the Cascades: snow. And snowpacks are declining.

“There are a lot of reasons that we want to increase snowpack, because that’s a natural storage capacity,” Nolin said. “And in many watersheds, the natural storage in the snowpack exceeds the storage in our reservoirs manyfold, like two or three times.”

But many factors can influence how much snow is stored as snowpack — including tree density. Even though trees shade snow, they also thin snowpack.

It works like this: Trees have dark canopies extending to the sky, absorbing sunlight like they’re supposed to. All that energy from the sun warms the trees, which, in turn, melts the snow near their trunks.

Trees also prevent precipitation from ever reaching the ground. Snow gets caught in the upper canopy, where it’s exposed to the sun and the wind. It evaporates before it can become part of the snowpack.

To examine the ways forest cover affects total precipitation, Nolin focused on one ecosystem: the Detroit Reservoir on the North Santiam River. Climate change has already dramatically altered the Santiam Watershed.

“We don’t have to look to the future to see declining snowpacks; we can see it in our own backyard,” Nolin said.

Recent droughts have caused the reservoir to empty entirely, leaving little water for people who live downstream or for salmon. Salem gets most of its drinking water from the Santiam. At Santiam Junction, near the headwaters of the North Santiam River, researchers have measured the snowpack on April 1 every year since 1941. In that period, snowpack has declined by 70 percent.

Adding or removing a single tree has very little impact on total snowpack, so to model the impacts of tree removal and fire, Nolin tested the extremes. She ran four scenarios in her model. In the first scenario, there was no logging in the forest above the Detroit Reservoir, and all fires were entirely suppressed.

Then Nolin ran models to see what would happen if every tree was removed above three different elevations: 1,500 meters (4,921 feet), 1,300 meters (4,265 feet) and 1,100 meters (3,608 feet). For each scenario, Nolin calculated the amount of water that would reach the watershed once the snowpack melts.

At very high elevations, where the trees freeze, this effect is less exaggerated: The trees are too cold to melt snow. Up there, shade from the trees can even keep snow on the ground longer. When all the trees were removed down to 1,100 meters, snowpack diminished significantly more.

Related: Snowpack Has Drastically Declined In The West This Century

This kind of research tells forest managers two things: first, that maintaining meadows and thinning dense forests can save water, and second, that logging at lower elevations adds more total snowpack than logging at higher elevations.

That doesn’t mean we should clear-cut the Cascades to stop drought, Nolin said. Healthy forests don’t just provide water; they provide work in the timber industry and create diverse ecosystems. And although trees warm the immediate area in the winter, they also cool it in the summer, providing shade and keeping streams healthy, clean and cold.

Instead, Nolin said, land managers and policymakers should focus on maintaining healthy forests. The thick, dense stands of replanted Douglas fir trees in second-growth forests aren't always healthy, and melt a lot of snow.

“If you thinned in that area, we found that you would accumulate more snow and retain it longer, and the forest might be a little bit less moisture stressed,” Nolin said. That’s a big deal, because previous models she’s worked on have shown a 200 percent to 900 percent increase in areas burned by forest fire on the western side of the Cascades by 2100.