Chinook are classified by when adults enter fresh water. Spring-run fish arrive sexually immature, months before spawning, and hold in cold headwater pools through the summer. Fall-run fish arrive nearly ripe and spawn soon after, lower in the system. The two share rivers but are separated in time and space.
Genomic work on chinook and on steelhead has repeatedly associated this timing difference with a narrow region of one chromosome containing the gene GREB1L and neighbouring sequence. Because the variation sits in a tight block rather than being spread thinly across the genome, early and late timing behave more like alternative alleles than like the ends of a smooth continuum.
That has a hard management consequence. If dams or warming remove the cold holding habitat that spring-run fish need, and the early-timed variant is lost from a basin, the remaining fall-run population cannot readily regenerate it. How quickly, or whether, the variant can re-emerge is genuinely uncertain, and it is contested whether early run timing warrants protection as a distinct conservation unit.