They don’t grow as big as salmon which go to the Pacific. Like all Pacific salmon, however, they still die after spawning.
The present thought is that after the last Ice Age, water levels were high enough to allow sockeye salmon to penetrate far up into inland water systems. As the water levels receded, or topology changed, access back to the ocean was blocked. The salmon learned to live and breed in inland water. Today, even though some streams do allow ocean access, and even though Kokanee salmon often meet up with ocean-going sockeye salmon in those streams, the Kokanee continue to be fresh-water fish only.
Kokanee” is a native word meaning “red fish”