The fish have developed lesions and a white fungus, a sign of thermal stress, after record-breaking torrid weather swept over the region.
The temperature in the Columbia River of 71F has exceeded the legal limit of 68, which scientists set to protect salmon from unsafe temperatures.
And conservationists are warning the tens of thousands of sockeye salmon remaining in the “dangerously warm” rivers could die as the waters grow even hotter in the next two months.
"Sockeye are dying right now because the Columbia and Snake rivers are too hot,” said Brett VandenHeuvel, executive director of conservation organisation Columbia Riverkeeper.
“I’m hopeful this tragedy will inspire our elected leaders to take action to restore our rivers before it is too late."
The salmon had been trying to return to the Snake River to spawn, when they unexpectedly changed course, he said.
The entire area is “superheated” because four manmade dams create unnaturally hot water thanks to huge stagnant reservoirs, and the climate crisis is pushing temperatures over the edge, the experts say.
Scientists predict that mass die-offs will become more common as the dams and the climate crisis continue to warm the rivers, sending Snake River sockeye salmon extinct, unless the dams are breached.
Columbia Riverkeeper says the video “provides a horrific glimpse of our future”.
The salmon were filmed with lesions and fuzzy-looking fungus as they took refuge in a cooler tributary of the river in an effort to essentially “escape a burning building”, Mr VandenHeuvel said.
The fish are expected to die of disease and heat stress without spawning. It’s too early to say how many will be killed.
He told The Guardian it was like a person trying to run a marathon in over 100F (38C) temperatures.
“The difference is that this isn’t recreation for the salmon,” he said. “They have no choice. They either make it or they die.
“It’s heartbreaking to watch animals dying unnaturally. And worse, thinking about the cause of it. This is a human caused problem, and it really makes me think about the future.”
Snake River sockeye, which are on the federal Endangered Species list, are “dangerously” close to extinction.
“We are losing so much more than a fish,” said Giulia Good Stefani, an attorney for the Natural Resources Defence Council.
“Salmon support a way of life for both native and non-native rural communities from the coast to the Northern Rockies.”