GRANITE FALLS — The Pilchuck River is getting warmer, and that’s not a good sign for future salmon runs.
The temperature of the water has risen about 5.4 degrees Fahrenheit above the historic norm, according to the state Department of Ecology.
Higher temperatures stress fish — including endangered species of salmon — and make them more susceptible to disease while affecting their reproductive system. The Department of Ecology released a plan this month detailing strategies to cool down the river.
Development is the main cause of warmer water into the river, Ecology watershed planner Heather Khan said.
A thick canopy of forest once shaded the Pilchuck River, she said.
Logging practices, agricultural uses and the conversion of land into hard, impervious surfaces like concrete all play a role in raising temperatures, she said. Climate change might play a role, but Ecology hasn’t studied that as a potential contributing factor.
Higher temperatures in the river slow down a fish’s swimming speed, making it more susceptible to predators, Khan said.
“If you can imagine heat stress and what that does to a person,” she said.
Warm water also has less dissolved oxygen, making it harder for fish to breathe.
The Department of Ecology has three strategies to cool down the river.
The first is to plant trees and native vegetation along the river and its tributaries. Once the plants mature, their shade will helps keep the water cooler.
“It’s like giving the stream a beach umbrella,” Khan said.
Ecology’s plan sets a goal of planting 381 acres of trees along the Pilchuck’s main stem by around 2061. That foliage could could reduce the main stem Pilchuck River’s temperature by 5 degrees over a span of 50 years. Plantings along additional streams will take until at least 2080. As the trees continue to fill out and grow taller, the benefits will increase, Khan said.
Ecology’s plan also calls for expanding cool pockets of water already found in the river.
The department started studying the Pilchuck River in 2012, and the research found cold-water refuges, sometimes almost 11 degrees cooler, throughout the river.
The pockets are often found where a tributary or groundwater enters the main river. And sometimes side channels have cooler water.
Ecology recommends expanding those habitats by placing large woody debris or boulders in the water, changing the water flow in a way that expands the area of cool water. That will create “stepping stones” for fish as they travel upstream, dashing from one pocket of chilly water to the next, Khan said.
Lastly, Ecology’s plan suggests ways to increase the flow of the Pilchuck during dry months.
Some tributaries dry up completely in the summer, blocking fish passage. And with less water headed for the main stem, the Pilchuck’s temperatures are driven higher.
There are a few ways to increase summer flow, Khan said.
The first is to conserve water so less is taken out of the river for commercial use.
Beavers could also help manage the flow by building dams.
Another way to increase summer flow in the river is to introduce stormwater runoff more slowly with swales or by planting rain gardens or using rain barrels.
Cities could also tap Spada Lake as a water source, through the city of Everett system, rather than taking it from the Pilchuck.
“We’re suggesting the counties and cities look at whether imported water makes sense for them,” Khan said. “We would have to be mindful of how much we’re pulling from the Spada Lake Reservoir and make sure we’re not overtaxing that.”
Ecology’s plan to cool the Pilchuck is open for public comment until Nov. 15.