The Raging River, a tributary of the Snoqualmie in the Cascade Mountains, was running swift in late June when a pair of researchers, Aimee Fullerton and Amy Marsha crossed the stream on a mission. They were there to retrieve water-temperature data from equipment on the opposite riverbank.

Wearing rubber waders to ward off the cold, the two researchers chose their steps carefully to keep from slipping on the moss-covered rocks. The rushing waters swirled around their legs, trying to upset their balance, but each had made this kind of trek many times before.

A sudden slip and fall into the water would have meant a cold shock for a human, even with protective gear. But for juvenile salmon swimming nearby, the 69-degree water had grown warm enough to cause stress and even inhibit growth. Worse, as adult salmon and steelhead arrive through the summer and into the fall the water could be warm enough in some places to kill them. The Raging River was already too warm for them in June, and it was destined to grow even warmer this summer.

In the Puget Sound region, elevated stream temperatures are believed to be one of the great downfalls for salmon, especially in areas where streamside vegetation has been removed by farming, forestry or development.

Fullerton and Marsha have been measuring and analyzing the changing conditions in the Snoqualmie and its tributaries as part of a major research project involving efforts from a dozen government and academic research groups. Coincidentally, their June visit to the Raging River and other locations came just one day before a historic three-day period of record-breaking heat throughout the Northwest.

Water temperature has a profound effect on the metabolism of fish, said Fullerton, a research fishery biologist with NOAA’s Northwest Fisheries Science Center. Temperature affects growth and development and plays a critical role in whether fish find food or get eaten by another animal.

Most people recognize that salmon need cold water. Elementary and junior high school students are taught that sunlight warms a stream, while streamside vegetation keeps the water cool. As a result, it is easy to believe that one portion of a stream could be too warm for salmon, another could be too cold, and another could be just right.

But such Goldilocks thinking has been greatly modified by careful studies into salmon behavior, as new technology measures stream temperatures with greater precision. Meanwhile, computer models are helping to account for shifting temperatures caused by changing streamflows, snowmelt, hot and cold springs, shade and air temperature, according to Marsha, a research engineer in the University of Washington’s School of Environmental and Forest Sciences who is focused on modeling efforts.

A growing understanding about how different salmon populations respond to stream temperatures offers hope for improved management of salmon habitat — now and into the future. With many populations of salmon already listed as threatened or endangered, new findings about temperature could provide ideas to help salmon survive the threats of climate change.

Gaming the system for survival

As cold-blooded creatures, salmon will swim from place to place to keep from getting too hot or too cold. They can generally tolerate less-than-ideal conditions for a time, especially when it comes to finding food and avoiding predators.

In Southwest Alaska, ecologist Jonny Armstrong, wearing a drysuit and equipped with mask and snorkel, entered the cold waters of Bear Creek, near Lake Aleknagik to observe the behavior of juvenile coho salmon. What he noticed over time was astounding.

During the day, the coho were not found where they were expected to be, that is in the colder sections of the stream where they could readily prey upon the nutrient-rich eggs of sockeye salmon. Instead, the “stunningly gorgeous and iridescent” coho were found schooling together in warmer water where food was less plentiful, said Armstrong, an assistant professor at Oregon State University who conducted these studies while working at the University of Washington.

“They looked kind of lumpy, so we knew they were eating salmon eggs,” he recalled. “In fact, they were completely full of salmon eggs.”

A young coho struggling to swallow a sockeye egg. In warmer streams, juvenile coho are more likely to grow large enough to exploit food subsidies from sockeye salmon. Johnny Armstrong

It turned out that the 3.5-inch coho would move downstream to the colder water when darkness fell. They would spend the night gorging themselves on sockeye eggs in waters cooled by springs. By morning, the young fish would return to the warmer waters, where the higher temperature raised their metabolism, allowing them to digest the food quicker and increase their body mass in preparation for the long Alaskan winter and a strenuous migration. Some of the fish would eat so many eggs that it would take two or three days to completely digest the meal, even in the warmer water.

“Anybody who does stream work soon learns that fish are amazing,” Armstrong said. “They don’t just accept the habitat they are given; they do all kinds of things to game the system.”

It isn’t clear why the coho chose to feed only at night, he said, but the day-night migrations may have reduced their risk of being eaten by birds or larger fish. Findings by Armstrong and his co-author Daniel Schindler were reported in the scientific publication Ecosystems.

In the Puget Sound region, elevated stream temperatures are believed to be one of the great downfalls for salmon, especially in areas where streamside vegetation has been removed by farming, forestry or development. Armstrong understands as well as anyone that high stream temperatures can result in serious problems for fish — including their ability to find prey or escape predators. But he worries that the issue has become over-simplified in the effort to protect salmon habitat.

It is much too easy, he said, for people to under-value habitat in sections of a stream that grow unacceptably warm for salmon during a few months of summer.

In an analysis published in Nature Climate Change, Armstrong and seven collaborators pointed out that maps of suitable habitat for salmon and trout are typically based on stream temperatures measured in the summer. That’s when most field researchers collect their stream data. Using summer temperatures, climate-change planners too often consider the warmest sections of a stream to be doomed when it comes to future salmon survival, the authors point out.

Armstrong’s analysis, including growth simulations in 14 watersheds, shows that downstream segments — where the water might grow too warm for salmon in the summer — could be safe or even ideal for their growth and development in the spring and fall, when the water is cooler. When downstream temperatures grow too warm, the fish might be able to find refuge in upstream waters cooled by springs or snowmelt.

Fish will move, seeking optimal conditions, Armstrong said. A key to understanding their behavior is to consider the effects of varying temperature on metabolism, prey and predators — not only during the summer but during all seasons. The essential need for salmon is to find cool enough water during critical times.

Too hot in the Snoqualmie

Back in the Raging River of the Cascade Mountains, Aimee Fullerton and Amy Marsha made it safely through the rushing waters. Fullerton unloaded a laptop computer from her backpack, while Marsha retrieved the data recorder from beneath a large rock holding it in place.

Working together, the pair downloaded near-continuous temperature data, taken in this location at 30-minute intervals. Such data, collected throughout the year for nearly a decade, will contribute to an ongoing story about the conditions faced by fish trying to survive, grow and migrate through less-than-ideal conditions in the Snoqualmie River.

In the Puget Sound region alone, the Washington Department of Ecology has identified more than 350 separate streams with serious temperature problems.

In all, NOAA scientists maintain 35 similar sensors dispersed throughout the Snoqualmie watershed, while King County researchers manage about the same number. Helicopters equipped with remote-sensing infrared technology have taken readings by flying over much of the Snoqualmie. Researchers have even paddled kayaks with temperature sensors in tow to help reconcile readings and explain the fate of cooler and warmer waters entering the river.

Location of Snoqualmie River basin in Washington (left) and map of water temperature monitoring sites in the basin (right). Map: King County Water and Land Resources Division

Reducing stressful temperatures for fish may require actions far more involved than planting trees to shade the stream, although shade is a basic consideration. Ideas include protecting or enhancing cold springs that provide safe refuge for fish endeavoring to survive warming waters.

Research now underway may inspire new ways of thinking about habitat restoration, not only in the Snoqualmie but in numerous other streams impaired by warm water. In the Puget Sound region alone, the Washington Department of Ecology has identified more than 350 separate streams with serious temperature problems. That number is expected to rise as climate change warms waters throughout the region.

The Snoqualmie River is part of the Snohomish River system, a lacy complex of streams that drains more than 1,800 square miles — from high in the Cascade Mountains through the lowlands and into Puget Sound near Everett. The Snohomish contains more than 1,730 tributaries for a total of 2,700 miles of streams. In terms of overall size, only the Skagit watershed to the north is larger within the Puget Sound region.

With eight species of salmon and trout in the Snohomish, experts have identified a variety of habitat problems over the past 50 years. Excessive temperature in many areas has become one of the most perplexing challenges, especially for the much-studied Snoqualmie River. Warm-water issues on the Snoqualmie begin well above the majestic Snoqualmie Falls, which drops 268 feet, forming a natural barrier to migrating salmon and steelhead.

The naming system for these rivers can be confusing, since the larger Snohomish River begins where the Snoqualmie and Skykomish rivers flow together. Upstream, the mainstem Snoqualmie begins near King County’s Three Forks Natural Area — two miles above Snoqualmie Falls. That’s where the North, South and Middle forks of the Snoqualmie come together.

The temperature of the Snoqualmie River — and ultimately the Snohomish — depends in large part on the size and temperature of upstream tributaries. The Middle Fork Snoqualmie — which provides between half and two-thirds of the combined flow of the mainstem — is the warmest segment. As a result, it creates a powerful influence on temperatures encountered by salmon downstream, as described in a report issued this month by the Snohomish River Basin Salmon Recovery Technical Committee.

Experts associated with the multi-agency Snoqualmie Science Coordination and Advisory Team (SnoSCAT) have been working together to unravel a variety of mysteries along the river — including the question of why the Middle Fork is so warm.

Some have speculated that the waters may be influenced by geothermal activity, since Goldmyer Hot Springs lies in the headwaters of the Middle Fork, said Andrew Miller, water quality planner for King County. Although warm water from deep underground has not been entirely ruled out as a factor, headwater streams near the hot springs are not especially warm, and their flows are but a “drop in the bucket” compared to the warm mass of water flowing downstream, he said.

Miller said more likely explanations for the higher temperatures in the Middle Fork are natural conditions, such as the topography of the river valley, as well as human changes to the channel during early periods of logging.

“Solar radiation has the power to warm the water,” he said. “In the case of the Middle Fork, we see warmer water in the lower reaches of the river where the channel widens out a lot.”

Warming grows pronounced where the Middle Fork assumes an east-west orientation, exposing the river to increased solar heating as the sun’s path follows the valley. Although trees have grown up since extensive logging in the 1900s, the channel itself may have grown wider and shallower as a result of sedimentation and other disturbances. If so, the result would be more water exposed to the warming sun. The channel also lacks natural complexity, which can keep the water cool but requires large trees falling into the stream over many years.

Human influences farther downstream in the Snoqualmie include unshaded stretches of river running through agricultural areas, as well as warmer water coming from sewage-treatment plants in the towns of Snoqualmie, Carnation and Duvall, all on the mainstem, and North Bend on the South Fork.

As in most river systems, the temperature of the river depends largely on the temperature of each tributary flowing in, as well as springs that feed cooler water into the system. Springs may emerge near tributaries or along the main river itself. When conditions are right, water can also pass through the riverbed and flow underground, often re-entering the river downstream. Called hyporheic flow, this subterranean excursion can exert a cooling effect on the river.

Studies in other river systems have shown that cool-water inputs, such as tributaries and springs, can provide a refuge for salmonids from the steady pace of warm water that can prove stressful or even deadly. In California’s Klamath River, juvenile Chinook and steelhead were found to consume a majority of their food in the inhospitably warm mainstem, from which they would retreat into cooler tributaries — namely Beaver, Grider, Fort Goff and Thompson creeks, according to a report by researchers at NOAA’s Southwest Fisheries Science Center.

During an extremely warm period in 2015, the mainstem Snoqualmie and major tributaries reached temperatures listed as “acutely lethal.” Safer temperatures were seen in several smaller tributaries and areas with cool groundwater discharges, which may have provided refuge for fish, although nobody can be sure.

Map of the Snoqualmie River basin with subbasins for each fork above Snoqualmie Falls highlighted by color (left). A time series of water temperature for the three forks and mainstem of the Snoqualmie River in 2015 (right). Colors from the time series correspond to the map on the left. Map and graph: King County Water and Land Resources Division

Later this month,  King County scientists Andrew Miller and Josh Kubo will put on their snorkel gear and enter the water to observe the behavior of fish in the mixing zones where cool tributaries and groundwater enter the Snoqualmie. In ongoing studies, the goal will be to better understand specific locations where salmonids are finding thermal refuge.

“This is a pilot study in which we will try to establish the methods (for research),” Miller said of this month’s dive trips. “We know in general that fish utilize the cold patches — but how can we best document where this is happening?”

To help salmon survive when the waters grow dangerously warm, it may be important to identify and protect these cooler areas and possibly enhance the habitat, such as with shade and woody debris, Miller said.

Others have suggested more costly solutions, such as placing structures called deflectors in the river to slow the mixing and enlarge the cold-water areas. Some have even raised the prospect of pumping groundwater to create cold-water refuges that might be needed to protect migrating salmon.

The strong influence of temperature

Many scientists argue that nothing is more important to a salmon than the temperature of the water. Temperature drives their rate of metabolism and determines how well they utilize food and oxygen, how fast they grow, and whether they have the energy to catch their next meal and avoid predators.

“Temperature is the master variable; it regulates their entire lives,” said John McMillan, longtime salmon researcher who now serves as science director for Trout Unlimited’s Wild Steelhead Initiative.

In the end, every fish faces constant tradeoffs involving a quest for optimal temperature, adequate food and enough speed and agility to escape from predators.

Every species of salmon is adapted to their individual life cycle and to the temperatures they will face from birth to death. Further, populations of salmon that grow up in a particular stream are likely to be adapted to the temperatures of that stream. Chinook in California, for example, may live normally at water temperatures that would be stressful or fatal to Chinook in Alaska.

Since warmer water speeds up metabolism and growth rate, one might expect that a year-old Chinook living in California would be larger than a year-old Chinook in Alaska. While true to some extent, the difference is less than expected, thanks to local adaptation that helps sustain long-term survival.

As McMillan explains it, faster growth in salmon leads to earlier physiological changes. Such changes can trigger migration before the timing is right for local conditions, such as food availability in the ocean. Too-rapid a growth rate can even lead to physical deformities. Genetic differences between fish from northern and southern waters establish separate growth rates appropriate for each population and the waters where they live.

When genetics works to counteract environmental effects, two or more populations living in different environments may end up more alike than if there were no genetic override. Transplant an Alaskan Chinook to California and it would likely grow much faster than its California cousin, while a California Chinook taken to Alaska would grow much slower, all because of temperature. This phenomenon, called countergradient variation, has its strongest effect when environmental differences are greatest.

Each population of fish has a metabolic optimal temperature at which the individual fish function best. At this temperature, they are best able to put out bursts of speed to catch prey or escape predators. If they can find food, they will have energy to grow larger, store fat for lean times and produce gametes for spawning.

Above the optimal temperature, the metabolism may run fast enough in an aerobic condition (with oxygen) that the fish’s organs may begin to have trouble getting enough oxygen. It doesn’t help that warmer water generally contains less oxygen. At still higher temperatures, or higher activity, the fish may need to switch to an anaerobic condition, using stored carbohydrates in place of oxygen. This is OK for short bursts of activity. But the fish needs to find cooler water or slow its activity — preferably both — to restore normal oxygen levels.

Scott Hinch, director of the Pacific Salmon Ecology and Conservation Laboratory at the University of British Columbia, describes fish that he saw in British Columbia streams in mid-July, following June’s regional heat wave. 

“The streams were 24 degrees (75 degrees Fahrenheit), and the juvenile chinook were barely able to move,” he said. “You could grab them with your hands.”

While the fate of various fish populations may be uncertain, this kind of temperature is likely to have severe consequences, according to Hinch. When the temperature gets too high, the metabolism speeds along even if the fish is not active. The body struggles to maintain basic functions. Oxygen and energy supplies are depleted. Stress hormones increase. And at a “critical temperature,” the organs shut down and the fish dies.

On the other hand, when the water is cooler than optimal, the metabolism of the fish is reduced. The fish moves more slowly. It can no longer act as quickly to catch food or avoid predation. Growth and development are slowed. This decline in function occurs more gradually at the cold end of the temperature scale, but eventually the metabolism drops below the baseline and becomes too slow to keep the fish alive.

Physiologists often talk about “aerobic scope,” which relates to the range of temperatures between the two critical temperatures — hot and cold — with the optimal temperature somewhere in the middle. Within its aerobic scope, a fish has metabolic capacity exceeding its basic needs — enough to support activity, growth and reproduction, all crucial to the survival of the species. When the temperature reduces aerobic scope to zero and the fish also runs out of anaerobic capacity, its life is over.

“It is one thing to sit motionless in a hot river,” Hinch said. “It is another thing when someone is chasing you and putting you further into oxygen debt. You don’t have any scope left.”

Research by Hinch and his colleagues found that female sockeye returning to the Fraser River of British Columbia were dying in numbers two to three times higher than males, leaving fewer females to spawn. That’s a major shift since the early 1990s, when females outnumbered males.

Higher female mortality, seen elsewhere in coho and Chinook salmon as well as sockeye, is believed to be the result of added stress from temperature and related factors, according to Hinch. Migrating females must divert blood flow to their eggs, which can deprive the heart and other organs of sufficient oxygen. Such stresses can also make the female more vulnerable to disease.

“Females are on a finer aerobic-scope scale,” said Hinch. “They are running out of energy, and their time is ticking. It gets to the point where their scope is exceeded.”

These findings, published in the Canadian Journal of Fisheries and Aquatic Sciences, have implications for populations of migrating salmon where high temperatures combine with other challenges to deplete energy reserves. Solving the environmental problems or at least reducing fishing pressure on the females are possible solutions.

Metabolic rate and suitable temperature range are different for each species of salmon and even populations within the same species. Because of their size and makeup, younger fish are able to tolerate a wider range of temperatures than adults from the same population. In the end, every fish faces constant tradeoffs involving a quest for optimal temperature, adequate food and enough speed and agility to escape from predators.

Warmed-over food supply

While some researchers in the Snoqualmie are analyzing the causes of temperature change in the river, others are studying the effects of temperature on the critical food supply for salmon and trout.

“The water can be as cold as you like, but if there is not food out there, the fish are not going to grow,” said Peter Kiffney, a research fish biologist with NOAA’s Northwest Fisheries Science Center. “We are missing a big part of the story if we don’t address the food supply.”

In fact, he points out, warmer water produces a higher rate of metabolism in fish, which means they need to find even more food to maintain their normal function.

Kiffney’s work includes wading into rivers and deploying a net to capture insects and other invertebrates floating down the river. The goal is to measure the size and type of food in the “drift” that is available for fish to eat.

A drift net, which captures insects and other invertebrates floating in a stream, is used by scientists to better understand fish diets. Peter Kiffney/NOAA

“Our studies are looking at what is in the drift and what is in the diet of fish,” Kiffney said.

Researchers are able to describe what the fish are eating under various temperatures and habitat conditions by pumping water into their stomachs and examining what comes out. This procedure allows researchers to safely release the fish back into the river. Such work is taking place not only throughout the Snoqualmie watershed but in many other streams throughout the Northwest.

“Juvenile salmon and trout are visual foragers,” Kiffney said. “They’re primarily eating invertebrates (such as stoneflies and mayflies). Some will eat small fish.”

Through careful studies, Kiffney and his fellow researchers are hoping to estimate the amount of food available to fish at a given time and place. This way they can measure the influence of overall food supply on the growth and survival of salmonids.

As the temperature goes up, invertebrates may also grow faster, provided they have access to their own foods, such as algae in the water. But if one part of the food web is missing or develops at the wrong time, it can throw off growth, migration or survival for salmon.

“We know the temperature is going up,” said Kiffney, “but we don’t really know how the invertebrates are going to react. I’m really curious to get at that question.”

Different populations of invertebrates emerge at different times, and some years are more productive than others for a variety of reasons. In certain periods, such as early summer, one may see “lots of bugs flying above the water,” as different species overlap in their development, Kiffney said.

As temperatures warm in a river, juvenile salmonids may look for cooler water in the upper reaches of a stream, closer to melting snow or glaciers. But will they find food in these areas? The answer may well depend on the availability of sunlight and nutrients to produce the algae needed to feed the invertebrates that feed the fish.

Unlike old-growth forests, some of the younger stands of managed forest far upstream contain buffers that, while keeping the water cool, block out sunlight. In such cases, making small cuts to open the tree canopy can improve productivity in the food web, Kiffney said. Meanwhile, it is well understood that most downstream areas are not lacking for sunlight, far from it.

Sometimes in these upper streams the problem is a lack of nutrients, which can be supplied by artificially adding salmon carcasses to mimic a productive salmon run, Kiffney said. These are temporary measurers at best, he added, and finding ways to restore salmon runs would be the best option.

At the same time, fish are able to eat terrestrial insects that get blown into the streams from surrounding vegetation. With higher nitrogen levels, terrestrial insects are often more nutritious than aquatic invertebrates.

When considering salmon recovery, a key question is what can be done to restore the food supply, including the possibility of planting trees or vegetation that can make the food web more resilient to climate change. It is a balancing act, scientists say, and studies that explain what food is available in streams and what fish are actually eating could help to answer questions essential to salmon recovery.

“We need some general strategies,” Kiffney said, “and we’re not there yet.”

Shining a light on the future

Thanks to a surge in technology, scientists are now able to map out temperature changes across an entire river system, such as the Snoqualmie. In the process, they are gaining an understanding about the stresses that fish must endure, especially during periods of extreme heat.

As a changing climate brings increasing stream temperatures throughout the Northwest, one challenge for humans is to help salmon escape from deadly temperatures, perhaps by securing cool-water refuges along their river route.

Using temperature as a surrogate for fish behavior, researchers can predict not only where fish in the river are likely to be, but also the best places for fish to hang out when no fish are present. 

For the past decade, NOAA’s Northwest Fisheries Science Center has been monitoring the stream temperature along the Snoqualmie and its tributaries. With 45 temperature data loggers taking measurements every half hour, researchers can piece together an overall story about how warm waters in one part of the river move downstream and mix with incoming flows to influence temperatures throughout the system.

Although 45 temperature sensors produce a lot of data — 2,160 readings per day — these sensors alone cannot reveal what is happening along the miles and miles of river between them. That’s where computer modeling and remote temperature sensing from aircraft can play a role.

One type of model, called a spatial stream network model, accounts for the branchlike network of tributaries that combine their flows as the river moves toward the sea. In a modeling project led by Amy Marsha, researchers incorporated 23 known characteristics of flow and land use throughout the Snoqualmie watershed. 

From the 45 sites where temperatures were measured precisely, the model estimated concurrent temperatures throughout the day for 1,219 closely spaced locations throughout the system.

Taking the project a step further, Marsha and her colleagues were able to describe where salmon and trout might spend the most time, given the changing water temperatures. They also located places where the fish might face peril from predators trying to eat them.

“These models are temperature based,” Marsha emphasized. “We can apply these models to inform us about fish behavior, but we are not modeling fish.”

While other researchers increase their efforts to study the behavior of fish directly or by tracking them with sophisticated “tags” embedded or attached to fish, such studies have proven to be challenging, expensive and geographically limited. Using temperature as a surrogate for fish behavior, researchers can predict not only where fish in the river are likely to be, but also the best places for fish to hang out when no fish are present. 

In a study published Feb. 12 in the journal Freshwater Science, Marsha looked for parts of the Snoqualmie River where suitable temperatures were seen for the various life stages of Chinook salmon, bull trout and largemouth bass — the latter a non-native fish found in the river and known to eat juvenile salmonids.

The model proved to be a reasonable way to search for places where salmon would likely hang out, based on temperature. It also revealed places where salmon would not like to stay. When combined with other habitat conditions, the model can point to places where restoration might make a real difference for salmon survival.

Another approach is to protect habitat where the temperature was shown to be suitable for salmon but too cold for warm-water predators, such as bass. In a similar fashion, one can now search for locations where conflicts between bass and salmon are most likely to occur.

“For example,” the report states, “we determined that major lowland reaches of the Snoqualmie River basin are thermally suitable for both native Chinook salmon and non-native largemouth bass during juvenile rearing.” Such areas may support increased predation of salmon by bass, increased competition between the species or reduced salmon growth and survival as a result of stress.

To better protect salmon in areas of potential conflict, habitat managers could propose targeted programs to reduce non-native bass populations, researchers say.

In a separate project that could help foretell the effects of climate change on the Snoqualmie watershed, Marsha joined a research team led by Ashley Steel of the U.S. Forest Service that looked into the extreme heat wave of 2015. Applying the spatial stream network model, they estimated temperatures throughout the Snoqualmie based on actual readings taken in 2015.

Their findings, published in the Canadian Journal of Fisheries and Aquatic Sciences, showed a remarkable divergence from typical  climate-change predictions, which often assume constant temperature increases throughout a river network.

As described in a 2018 article about the research, “Some locations showed dramatic increases in air and water temperature, whereas others had temperatures that differed little from typical years; these results contrasted with existing forecasts of future thermal landscapes.

“If we will observe years like 2015 more frequently in the future, we can expect conditions to be less favorable to native, cool-water fishes such as Chinook salmon and bull trout but beneficial to warm-water nonnative species such as largemouth bass.”

Climate change is expected to bring more frequent heat waves, such as the one in 2015 and again this year — although warm-water conditions this summer have not matched those of 2015, despite the record-breaking air temperatures. One difference may be the increased snowpack over the past winter. Still, climate change is expected to bring an increasing frequency of extreme weather, and stream models could reveal just how harsh conditions may get in the Snoqualmie and other systems where studies are taking place.

Speedy measurements and next steps

Another technology that adds a powerful new dimension to stream-temperature studies is thermal infrared sensing using equipment mounted on small airplanes and helicopters. Remote sensing can provide colorful images of a river, with red and orange blotches often representing warm water and whisps of turquoise and blue accentuating cooler flows coming from tributaries and springs. 

In the Snoqualmie, a helicopter carrying thermal imaging equipment was able to cover the entire river in a single day in August 2020. This kind of equipment detects infrared radiation coming off the water, and those measurements can be converted to temperature.

View of Snoqualmie Falls from a helicopter conducting remote-sensing operations for temperatures along the Snoqualmie River. Jared Ritchey, Quantum Spatial

Remote sensing is less precise than a sensor emersed in river water; single flights provide just a snapshot of temperature; and the measurements can be affected by atmospheric conditions.  And yet the ability to survey an entire river without hiking into the watershed provides a valuable way to map out the temperatures along an entire river, according to Dan Restivo, hydrologic technician for the U.S. Geological Survey who is leading the project. 

When the remotely acquired data are analyzed, they can be calibrated with temperature readings taken from the water itself, effectively connecting the dots to provide more accurate readings for the entire river, he noted.

The project in the Snoqualmie is evaluating not only the benefits of using a helicopter for remote sensing but also the potential of unmanned aircraft (drones) to gather temperature data, Restivo said. Despite higher operating costs per hour, the helicopter can fly at higher elevations and cover the entire river in one swoop — even it if has to return briefly to complete the temperature profile. 

Drone operators, on the other hand, must keep their unmanned aircraft in sight, which means a lot of travel over the ground. Still, one major benefit of the drone is a higher resolution of the temperature differences in the river: about 8 by 8 inches for each reading from the drone used in the Snoqualmie versus about 20 by 20 inches from the helicopter.

Another helicopter run was completed in early August. Data from the flight are now being analyzed. One anticipated outcome is an improvement in the techniques used to measure stream temperature, not only in the Snoqualmie but anywhere similar studies are conducted.

One intriguing question waiting to be answered is whether the cool tributaries and springs observed in the fly-over last year were still there a year later, said Restivo along with Matt Baerwalde, environmental policy analyst for the Snoqualmie Tribe who has been studying the effects of stream temperature on the entire ecosystem.

“Persistence of cold-water tributaries is really important,” Baerwalde said. “The next step is to figure out the mechanisms that support these cold-water features so that we can protect them.”

In some cases, cold water may be entering the river, not through a single spring or tributary, but in groundwater flowing into the river for a mile or more, he said. Understanding such flow — perhaps by installing groundwater-monitoring wells — could be critically important. That’s because human activities — such as heavy groundwater pumping — could disrupt the flow and raise the temperature of the river in these vital areas.

The Snoqualmie Tribe is using computer models to study future stream conditions out to the end of the century, considering potential changes in climate and land use — including the size and condition of riparian (streamside) buffers that shade the river.  

“It gets really complicated with all these different climate scenarios and riparian land-use scenarios,” Baerwalde said.

Although large buffers would help, even those going back 100 meters (328 feet) from the water would not entirely compensate for the higher temperatures expected from climate change, he said. 

The tribe has been working with farmers in the Snoqualmie valley to find practical solutions to the temperature problem, he said. In a key finding from the modeling effort, the tribe concluded that 30-meter (98-foot) buffers worked almost as well as those 100 meters wide.

“We understand that it might be impossible to achieve 300-foot buffers everywhere,” Baerwalde said, “but we need to get buffers planted now.”

Using another model in partnership with the Environmental Protection Agency, the tribe found that young stands of Douglas fir trees drink a lot of water. A counter-intuitive solution for protecting groundwater flow might be to thin the young forest and perhaps grow marketable trees that don’t need as much water. 

Baerwalde is also studying methods of forest management that might retain mountain snow on the ground longer into the spring and summer to help reduce peak water temperatures.

“There are better ways to manage forests that take into account the temperature of the river,” he said. “The market is a key to the solution. The more questions we answer, the more there are. But figuring it out is what gets me excited to go to work.”