Winter-run Chinook salmon spend 1–3 years in the ocean. Adult winter-run Chinook salmon leave the ocean and migrate through the Delta into the Sacramento River from December through July with peak migration in March (Moyle 2002). Many of the early-arriving fish hold for extended periods in the river prior to spawning. Spawning occurs from mid-April to early-August with egg incubation occurring from mid-April to September (Vogel and Marine 1991). The primary spawning habitat in the Sacramento River is above Red Bluff Diversion Dam (RBDD) at river mile (RM) 243, although spawning has been observed downstream as far as RM 218. Spawning success below RBDD may be limited primarily by warm water temperatures (Hallock and Fisher 1985).
Downstream movement of winter-run Chinook salmon fry begins in August, soon after
emergence from the gravel. The abundance of juveniles moving downstream peaks at Red Bluff in September and October (Vogel and Marine 1991) but the lower river reaches are generally too warm for fish to enter the Delta at that time. Juvenile Chinook salmon move downstream from spawning areas in response to many factors, which may include inherited behavior, habitat availability, flow, competition for space and food, and water temperature. The numbers of juveniles that move and the subsequent timing are highly variable. Storm events and the resulting high flow and turbidity along with cooler water temperatures appear to trigger downstream movement of substantial numbers of juvenile Chinook salmon. In general, juvenile abundance in the Delta increases in response to increased Sacramento River flow (USFWS 1995). The peak movement of juvenile winter-run salmon into the Delta occurs during the winter months, but winter-run Chinook salmon smolts (i.e., juveniles that are physiologically ready to enter seawater) may migrate through the Delta and bay to the ocean from November through May (Yoshiyama et al. 1998). The Sacramento River channel is the main migration route through the Delta. However, the Yolo Bypass also provides significant outmigration passage during higher flow events. During winter in the Sacramento Valley, juveniles rear on seasonally inundated floodplains. Sommer et al. (2001) found apparent higher growth and survival rates of juvenile Chinook salmon that reared in the Yolo Bypass floodplain than in the mainstem Sacramento River.