What do heat-stressed salmon, gluttonous sturgeon, swimmer’s itch and class-action law suits have in common? If you answered warm water, go to the head of the class. If you answered climate change, you might also be correct, but only time will tell. One thing is for sure: Pacific salmon are in trouble unless conditions improve during the fresh water stage of their life cycle.

Elevated river water temperatures created front-page news in the summer of 2015 when low stream flow and an extended heat wave created a perfect storm for Columbia River fish. The upriver sockeye salmon run was decimated. Over 80 mature white sturgeon died after eating dead sockeye and suffering “metabolic overload” due to heat stress and insufficient dissolved oxygen to fuel digestion. A major die-off of pre-spawning spring-run Chinook salmon occurred in the Middle Fork John Day River.

These incidents and others quickly spurred action. The conservation group, Riverkeeper, sued the EPA saying they “weren’t doing enough to protect salmon from high water temperatures” on the Columbia and Snake Rivers. Several fishing advocacy groups and the Idaho Rivers United joined the fray to fight for cooler water. A 2017 report released by Riverkeeper blamed hot water caused by dams for killing salmon. Their computer model showed that water temperatures would be cooler if the four lower Snake River dams were removed. High temperatures were largely attributed to slow-moving reservoirs that “soaked up the sun.”

A Brief History Lesson in Water Temperature

Over two centuries ago, on October 17, 1805, Captains Meriwether Lewis and William Clarke arrived at the confluence of the Snake and Columbia River. Along with the general lack of wood, they noticed large numbers of dead salmon “on the shores and floating in the river….” . Clarke expressed his befuddlement at the sight in a brief journal passage, “the Cause of ….I can’t account for.”

What Lewis and Clark documented was the end of the fall Chinook salmon spawning period. Not being trained in ichthyology, they had no knowledge of the salmon life cycle. Dead and dying salmon were not a natural occurrence to them. The significance of their account is that fall Chinook salmon now spawn in this part of the Columbia River (i.e., the Hanford Reach) nearly a month later, primarily due to a phase shift in seasonal water temperature. The lag is largely due to water storage practices of Columbia River dams located high in the watershed.

Another temperature-related fact was documented on August 15, 1811 when the explorer Alexander Ross stood at the junction of the Snake and Columbia River and noted, “Lewis’s River (Snake) has a muddy or milk-and-water appearance and is warm, while Clarke’s River (Columbia) is bluish, clear, and very cold.” What Ross’s note suggests, contrary to popular opinion, is the lower Snake River has always been warmer than the Columbia.

Fast-forward to the 20th century when 11 and 8 mainstem dams, respectively, were constructed in the Columbia and Snake River watersheds. Their construction blocked salmon and steelhead access to upriver production areas and dramatically altered flowing habitats to a series of slackwater pools. Subsequent operation of these dams (and hundreds of others built in tributary streams) has modified seasonal patterns of temperature and flow.

Fish Physiology and Water Physics

How does warm water affect salmon and steelhead? First and foremost, fish are cold-blooded. This means their body temperature approaches that of their environment. Every fish species has an optimum temperature zone where growth, reproduction, and other activities are conducted most effectively. Beyond this zone are unfavorable high or low temperature regimes. Salmon and steelhead favor cooler water than so-called warm water-tolerant bass and catfish. The upper inflection point, beyond which salmon and steelhead are adversely affected, is considered to be around 70 degrees. Field and laboratory studies suggest they seek lower temperatures, typically 55 to 60 degrees, if given a choice.

The sun is the primary source of heat that warms natural water bodies. Thus, seasonal temperatures follow a distinct pattern relating to day length and angle of the sun. Heat loss occurs from rivers due to evaporation and exchange with air and landforms, making it difficult to trace gain and loss from a particular source. As an example, nuclear reactors released thousands of gallons of hot water into the Hanford Reach of the Columbia River in the 1960s. Monitoring studies showed that heat dissipated quickly due to mixing and atmospheric exchange. Cold-water releases from the bottom-layer of storage reservoirs are also moderated over distance. Where warm and cold water mix, for example at the confluence of rivers, the denser cold water source sinks. These same phenomena can lead to a warm surface layer forming in Columbia and Snake River reservoirs. Fish respond by selecting a preferred temperature.

Another important feature influencing water temperature is volume. Large water bodies have more stable temperatures, while small shallow lakes and streams show greater fluctuation in temperature, particularly on a daily basis. Stream length is also a factor. The longer the river the more thermal units absorbed over the course of a day. Daily temperature swings in the Deschutes River between Whisky Dick and Moody Gauge, a distance of almost 80 miles, can range up to 6 degrees in the summer. Similarly, the Columbia River increases in temperature from its origin high in the Rockies to where it enters the Pacific Ocean. Much of the warming is consequence of shallow water heating up in the daytime.

One reason salmon and steelhead seek cool water respite is because the amount of oxygen in water at equilibrium goes down as temperature goes up. Most fish compensate for a drop in oxygen concentration by increasing their ventilation or breathing rate and increasing cardiac output. There’s a limit to how long they can compensate without being overtly stressed, though. Because adult salmon and steelhead do not actively feed once they reach freshwater, the stress of dealing with elevated water temperature along with migration can burn up their valuable energy reserves.

Cold Water Refuges

Having access to cool water during the freshwater migration phase increases the likelihood that summer-migrant salmon and steelhead will successfully complete their life cycle. To address that issue, the EPA currently leads a study to identify and evaluate cold water refuges from the mouth of the Columbia River upstream to the Oregon-Washington Stateline, a distance of 310 river miles. Most refuges are found upstream of the confluence of major tributary streams. (Learn more about EPA’s Columbia River Cold Water Refuges Project at https://www.epa.gov/columbiariver/columbia-river-cold-water-refuges.)

Many of these same confluence areas are popular with anglers, including Herman Creek, Wind, Little White Salmon (Drano) and the Deschutes Rivers. The Deschutes River provides the largest volume of cold water relative to other Columbia River tributaries. Influx of cooler water is one reason fall Chinook salmon stack up “like cordwood” off the tributary mouth, attracting boatloads of anglers in late August and early September. Just downstream, steelhead anglers anchor up in Drano Lake where cool water settles in the old river channel. Upriver anglers benefit from a thermal block that occurs at the mouth of the Okanogan River, where summer Chinook and sockeye salmon rest in Columbia River water as much as 8 degrees cooler.

Upstream of the Deschutes, few tributaries have sufficient cool water flow to reduce Columbia River water temperature until fish reach the Wenatchee and Clearwater Rivers. It’s particularly important for steelhead to take advantage of cold water refuges because they don’t spawn until the following spring. Tributary overshoot, is common for summer steelhead whose home stream is the John Day, Umatilla and Walla Walla Rivers. These smaller streams are generally warmer than the mainstem Columbia in late summer.

Summer Chinook and sockeye are on the fast track to upriver spawning grounds with little time to waste in the lower Columbia. Radio-tracking studies show that Chinook don’t generally use coldwater refuges until water temperatures reach 70 degrees. In the Fraser River, sockeye show a range of thermal tolerance, depending on upriver migration distances that vary from 150 to 1100 miles. Columbia and Snake River sockeye have no choice but to spawn high in their respective watersheds, which limits their ability to adapt to changes in water temperature.

Want to put more fish on your catch card? Having access to a temperature probe will help you locate favorable depths for salmon and steelhead when surface water temperatures approach or exceed 70 degrees. When in doubt, keep your lure near the bottom where cooler temperatures prevail.

What Will the Future Hold?

Climate change will continue to affect fish that live in streams and rivers in different ways, depending on regional geography. For example, streams flowing from the west side of the Cascade Mountains are largely driven by rainfall. In contrast, stream flow on the eastside depends mostly on the amount of snow pack and timing of its release. Because of long-term changes in climatic conditions, many northwest rivers appear to be warming earlier and staying warm longer. This condition provides less of a challenge for fishes having the ability to move to cooler headwaters than those that must migrate long distances to complete their life cycle.

In response to temperature issues, fisheries managers and action agencies have initiated several actions to aid migration of salmon and steelhead during periods of high water temperature. During the drought year of 2015, biologists trapped Sockeye salmon at Snake River’s Lower Granite Dam and hauled them to spawning grounds in Idaho. The U.S. Army Corps of Engineers provided cold-water releases from Dvorshak Dam on the North Fork Clearwater River and modified operations at Lower Granite Dam that freed up cooler water from deep spill bays. In the Yakima River, the Bureau of Reclamation provided an extra slug of water from upstream reservoirs to promote salmon entry into warm, slow-moving water where the Yakima meets the Columbia.

All fish are vulnerable to heat stress if they struggle to the point of exhaustion. For example, it takes as much energy for 20 seconds of burst swimming activity as for 15 minutes of sustained swimming or three hours of inactivity. Under certain conditions, extended swimming will lead to oxygen debt. For those reasons, anglers should land all salmon and steelhead quickly if they are inadvertently hooked and where catch-and-release regulations are in effect. Keeping fish in the water until they regain the strength to swim away safely is another good practice.

Negotiations are currently underway to determine the future of a half-century-old agreement between the United States and Canada that governs Columbia River flows. Modernizing the Columbia River Treaty to include specific fish and wildlife concerns could go a long way towards improving the summer-time environment for upriver populations of salmon and steelhead. Meanwhile, the identification and maintenance of cold water refuges remains an important management tool for ensuring the long term survival of these iconic fishes.