Warm Deschutes good for playing, bad for fish
七月 23, 2017
River temperatures rise as the summer progressesAsk any Central Oregonian how to beat the heat during the summer, and odds are good the Deschutes River will be part of the solution. The 252-mile river that cuts a swath through the center of Oregon — and Bend — has a reputation for providing an escape for locals and tourists alike to float, paddle and swim.
However, the conditions that make the Deschutes River comfortable for summer recreation have also made it unbearably hot for native salmon and trout populations during the late summer months, thanks to rising outdoor temperatures, increased amounts of sediment in the river and the presence of wide, shallow reservoirs at points along its winding course.
“It’s probably warmer than it was historically, and it’s probably warmer than it should be,” said Ryan Houston, executive director of the Upper Deschutes Watershed Council, a nonprofit group that focuses on restoring habitat along the Deschutes River.
The Oregon Department of Environmental Quality tracks bodies of water that fail to meet requirements for temperature, water quality and other elements, in compliance with the federal Clean Water Act.
Don Butcher, water permitting manager with the department, said river miles 110 to 223 — from Wickiup Reservoir to well north of Bend — failed to meet water quality standards because they exceeded 18 degrees C, or roughly 64 degrees F, at points during the summer.
While the Deschutes River is not the only river in Oregon or Central Oregon to exceed the mandated temperatures, Butcher said having river water that’s that warm can make it difficult for fish species like redband and bull trout to spawn and eat effectively.
“One of the most difficult things for fish to deal with is heat,” Butcher said.
The river takes a circuitous path from Little Lava Lake in the Cascade Mountains to Lake Billy Chinook, where the river meets the Crooked and Metolius rivers and eventually connects with the Columbia River. Even in the Upper Deschutes, which runs from the headwaters near Little Lava Lake to the city of Bend, the average daily temperature can vary by more than 10 degrees at different locations along the river, depending on the elevation and flow of the river, as well as how much shade it receives, according to Kyle Gorman, region manager for the Oregon Water Resources Department.
“It’s a really interesting stream flow,” Gorman said.
Crane Prairie Reservoir, southwest of Bend, is a wide, shallow reservoir, with a maximum depth of 20 feet, according to Tom Walker, fisheries biologist for the Deschutes National Forest. Because of that, the water sitting in the reservoir receives a lot of sun before it continues on its journey, which can raise the average temperature above 70 degrees during the hottest part of the summer.
“It’s like bathwater coming out of there,” Houston said.
Walker said the rising temperatures limit the amount of oxygen in the water, which has a negative impact on fish populations that are native to the Upper Deschutes, including redband trout.
“Fish are adapted to a certain temperature,” Walker said. “When it’s beyond that, their metabolism changes.”
In warmer water, redband trout are at a disadvantage to brown trout, a larger, invasive species, he said. Brown trout prey on the smaller native fish and outcompete them in the Deschutes River.
Houston added that bull trout, historically part of the Upper Deschutes ecosystem, no longer live in that part of the river because they can’t cope with the hot temperatures.
“The impact of warm water is that we tend to lose species,” Houston said.
Houston added that Crane Prairie drains from the top of the reservoir, meaning that the warmest water continues along the river. By contrast, Wickiup Reservoir, a larger man-made lake downstream from Crane Prairie, drains from the bottom, meaning that water temperatures downstream can be more than 10 degrees lower.
Houston said the streamflow gets more complicated as the water makes the trek toward Bend. It meets up with several rivers with colder water that are fed by underground springs, including the Fall River. However, the Little Deschutes River, one of the larger Deschutes’ main tributaries, tends to run warm, which Houston said was thanks to warm discharge from Gilchrist Log Pond and the languid pace of the river itself.
“It’s a very flat, low-grade, sinuous river,” he said of the Little Deschutes.
By the time the Deschutes River reaches the Old Mill District — the stretch of the river best known to floaters and paddlers, it tends to warm back into the high 60s during the hottest part of the summer, according to data provided by the Upper Deschutes Watershed Council. It continues to warm at Mirror Pond, where the City of Bend’s system of dams slows the water and leaves it exposed to more sunlight.
Houston added that Tumalo Creek, which meets up with the Deschutes River northwest of Bend, can either warm or cool the Deschutes, depending on how much water farmers divert.
In 2012 the Department of Environmental Quality’s designation of parts of the river that are too warm for salmon and trout triggered a notice to land management authorities along the Deschutes River, requiring them to look at ways to reduce the temperature of the river.
Walker said one solution is planting willows and other plants along the banks of the river to reduce the amount of sediment entering the river and increase the amount of shade. Eliminating user-created trails, which push dirt into the river, and keeping an eye on the amount of water flowing through the river are other solutions.
“Managing flow is the biggest issue on the Deschutes right now,” Walker said.
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