Steelhead trout are defined by the California Department of Fish and Wildlife as any rainbow trout greater than 16 inches in length caught in anadromous waters.
It is only legal to keep hatchery steelhead and only in select waters, see https://www.wildlife.ca.gov/Regulations
Hatchery steelhead are identified by a missing adipose fin and healed scar in the location of the missing fin. All wild steelhead must be released unharmed.
Check California Department of Fish and Wildlife Regulations https://www.wildlife.ca.gov/Regulations for legal sizes, bag and possession limits, and seasons, which can vary from year to year.