Algae in Iron Gate Reservoir © EcoFlight

By K.C. Mehaffey
Sep 8, 2025

Water temperatures in the Klamath River are responding to last year’s removal of four hydroelectric dams in ways that scientists say are beneficial to salmon, steelhead and other aquatic life.

Researchers and salmon managers are also seeing a lower prevalence of Ceratonova shasta (C. shasta), a parasite that has plagued juvenile salmon downstream of the stretch of river where the dams were removed (Clearing Up No. 2006).

Outbreaks of harmful algal blooms that prompted public health advisories are smaller and less frequent.

“If the dams remained in place, in the face of climate change all of those water quality impairments would have gotten worse,” said Crystal Robinson, Klamath Watershed program manager for the California Department of Fish and Wildlife.

“We’re just opening up that river to be free-flowing, and basically just allowing it to do its normal hydrological things: scour the river, help with fish disease, help with the temperature aspect and get rid of blue-green algae,” she told Clearing Up.

The four dams—Iron Gate, Copco 1, Copco 2 and J.C. Boyle—were in a 38-mile stretch of the Klamath River, and their reservoirs covered about 2,200 acres of land.

In November 2023, the Klamath River Renewal Corporation took over the license of the dams [P-14803] from PacifiCorp after the Federal Energy Regulatory Commission approved the license surrenders and plans for the dams’ removals.

Last year, KRRC began drawing down the reservoirs in January. They were drained in time for spring runoff.

The removal work was completed in September 2024, and a contractor—Resource Environmental Solutions (RES)—oversees the multiyear restoration.

In its first year as a free-flowing river, scientists are already seeing dramatic changes in average daily water temperatures in the stretch of river. In general, the river warms up sooner in the spring, cools off sooner in the fall, and has much greater fluctuations between daytime and nighttime temperatures throughout the year.

Average daily Klamath River temperatures at the Iron Gate gauge. Resource Environmental Solutions

Caitlin Boise, the Klamath project’s water quality technical lead for RES, and Dan Chase, director of fisheries, aquatics and design for RES’ Western Region, teamed up to answer Clearing Up’s questions about the importance of the temperature changes in the Klamath River, and what they mean to salmonids and other aquatic life.

“Temperature influences nearly all chemical, physical, and biological processes in rivers,” the RES team told Clearing Up in an email. “These can include everything from the amount of oxygen the water can hold, to the rate of chemical reactions like decomposition of organic material, to the habitat that is actually available to fish and other aquatic species, to the speed at which fish and other aquatic organisms grow.”

In 2024 and 2025, temperatures at the former Iron Gate Dam reached 50 degrees Fahrenheit about one month earlier compared to 2023—the year prior to the drawdowns and dam removals.

Warmer spring water temperature can boost growth for emerging salmonids rearing in the river, according to a KRRC newsletter.

Robinson noted that means these young fish are ready to migrate downstream sooner, diminishing their chances of interacting with the C. shasta parasite.

She said the prevalence of C. shasta in juvenile salmon was lower this year compared to previous years.

Robinson said in the summer, the slow-moving reservoirs created an environment that allowed blue-green algae to thrive. For several years, parts of the Klamath River have been posted with public health warnings for people and pets to stay away from the toxic algal blooms.

“We’ve eliminated that public health threat,” she said, adding that fish exposed to the algal blooms can also have high levels of toxins.

Conditions are also better for fish in the fall.

In 2024, the water at the Iron Gate gauge cooled about a month earlier compared to 2023, reducing the potential for disease and thermal stress. Cooler water can also be a cue for migration and spawning, the newsletter noted.

“Basically, the reservoirs were creating conditions for fish where the temperatures were inhospitable during migration,” Robinson said.

She said this year, during the first week of September, a heat wave prompted salmon and steelhead migrating up the Klamath River to hold in place at the mouth of the Salmon River, where colder water was coming out of that tributary.

But as air temperatures cool back down, the river upstream will respond quickly, convincing the fish to continue their migration, she noted.

“That’s one of the things that we can see from the data that’s changed,” Robinson said.

Another benefit to fish is the daily fluctuations in temperature throughout the year.

In 2024, the average daily fluctuations at Iron Gate increased to about 5 F, compared to 1.75 F in 2023, and similar results are expected once the full dataset is available this year.

These fluctuations are important to native fish and salmonids because it gives them options, the RES team said.

“Cooler temperatures at night in a healthy river allow fish to more freely and easily move around the system. This increases the area they have access to forage as they are no longer restricted to small pockets of temperature refugia that remain isolated through the night. This also allows fish to redistribute and can help with density-related pressures like food availability and disease burden,” the team said.

The RES team said that Iron Gate is the point of comparison because it was the compliance dam for the Lower Klamath Project and has a long-term record. It was also “the end of the road for fish and now it’s the open gate.”

However, the team is seeing similar changes in temperatures downstream of the Copco 1 location, with temperatures warming up to a month in the spring, and cooling up to a month earlier in the fall, and daily fluctuations of about 4.8 F.

“The reach downstream of J.C. Boyle is unique in that there are a series of naturally occurring cold-water springs that make this one of the coldest stretches of river,” they noted, adding, “This is a crucial benefit of the project: fish again have access to this cool, high-quality habitat for the first time in over one hundred years.”

It’s not too soon to compare temperatures from before and after the dam removal, they said. And—with the massive restoration work to replant native grasses, trees and other plants, they’re expecting to see these temperature changes improve as the vegetation matures.

And while temperatures have improved in the stretch of river where the dams were removed, warm water is still coming downstream from Keno Dam. However, cold-water contributions below Keno dam—like J.C. Boyle Springs and Fall Creek—are no longer being lost and warmed in the reservoirs, the team said.

Along with passage—which did not exist while the dams were in place—the improved temperature regime and other environmental changes are expected to help salmon, steelhead and other native fish recolonize the upper Klamath River now, and in the years to come.

“We’re only 11 months past the completion of dam removal, and only several months since the first cohort of fish spawned in the newly reconnected habitat,” the RES team noted.

These salmonids now have hundreds of miles of habitat for adults to spawn, and for juveniles to feed and grow. And with the removal of reservoirs that provided habitat for nonnative fish, removing the dams also prevents some of their competitors and predators from continuing to thrive, they said.

News Data: Klamath River Water Temperatures Responding to Dam Removal