Hatcheries release them, according to this report from KCRA 3.
“GOLD RIVER, Calif. —
“Steelhead season is underway in the Central Valley as three major hatcheries are set to release over 1.1 million fish into the Feather, American and Mokelumne rivers later this month.
“Steelhead are the migratory form of rainbow trout that make their journey to the Pacific Ocean and return to freshwater streams.
“Unlike salmon, steelhead don’t die after spawning and can make the journey from freshwater to saltwater and back multiple times in their lives, according to the California Department of Fish and Wildlife.
“Three CDFW-operated fish hatcheries in the Central Valley are set to release the next generation of steelhead later this month. More than 1.1 million fish combined will be released into the American, Feather and Mokelumne rivers.
“The year-old steelhead may travel more than 100 miles to reach the Pacific Ocean, returning in two to three years to spawn in the rivers from which they were released.
“Fish biologists with the department say other steelhead may spend their entire lives in the rivers much like the resident rainbow trout.
“The Feather River and Mokelumne River hatcheries release a much smaller Central Valley species of steelhead that can reach up to seven pounds. The Nimbus Fish Hatchery in Gold River release a coastal species of steelhead that can grow to 20 pounds in size.”
Retrieved February 19, 2021 from Millions of steelhead to be released throughout Central Valley (kcra.com)