How Long Before These Salmon Are Gone? ‘Maybe 20 Years’

As they travel, the parrs, or young freshwater salmon, undergo a profound transformation called smoltification, becoming smolts able to thrive in saltwater. After leaving the river, the fish turn north and travel to the North Pacific, near the Aleutian Islands.

They spend up to four years feeding at sea, and then those that survive the seagoing journey return to the mouth of the Columbia. Their physiological changes are reversed as they move upstream, and they again become freshwater fish.

Picking up the scent of their natal stream, they fight the current, foregoing food on the grueling trip, gaining about 6,500 feet in elevation, and overcoming physical barriers in what biologists describe as a heroic journey.

“I’ve seen them jump an eight-foot waterfall, and they are known to jump 12 feet,” said Mr. Thurow. “They are the definition of persistence.”

Chinook are known as “high-fidelity” spawners, not only returning to the stream where they were born, but also often to the same shallows. Then the game is afoot: In their waning days, as males battle for dominance, females excavate a redd, a depression in the gravel riverbed.

The female releases clusters of eggs as the male sidles up, releasing its sperm at the same time. The current mixes them, resulting in fertilization. The eggs are adhesive and stick to the gravel after they fall. The female buries them in an egg pocket.

The mating is repeated multiple times; all told, some 5,000 eggs may be released by a single female. “By the times she finishes, she’s within a day or two of dying,” Mr. Thurow said. The next spring, the offspring emerge and make their own journey to the sea. Always a gauntlet, the migration now is far more deadly. The eight large dams along the Snake and Columbia rivers created 325 miles of slack water in reservoirs. The average speed of the water flowing downstream has dropped to less than 1.5 miles per hour, and it takes the fish far longer to reach the sea.