Spring-run Chinook was believed historically to be the most numerous salmon stock in the Central Valley prior to construction of many Central Valley dams, which blocked access of the fish to their historical habitats. Only the main-stem Sacramento and some of its tributaries support remnant spawning runs. Adult spring-run Chinook salmon enter the mainstem Sacramento River from March through September, with the peak upstream migration occurring from May through June (Yoshiyama et al. 1998). Spring-run Chinook salmon are sexually immature during upstream migration, and adults hold in deep, cold pools near spawning habitat until spawning commences in late summer and fall. Spring-run Chinook salmon spawn in the upper reaches of the mainstem Sacramento River (downstream of Keswick Dam) and tributary streams (USFWS 1995), with the largest tributary runs occurring in Butte, Deer, and Mill creeks (Yoshiyama et al. 1998), the Feather River downstream of Oroville Dam, and the Yuba River downstream of Englebright Dam. Spawning typically begins in late August and may continue through October. Juveniles emerge in November and December in most locations, but may emerge later when water temperature is cooler. Newly emerged fry remain in shallow, low-velocity edgewater (DFG 1998).
Juvenile spring-run Chinook salmon typically spend up to one year rearing in fresh water before migrating to sea as yearlings, but some may migrate downstream as young-of-year juveniles. Rearing takes place in their natal streams, the mainstem of the Sacramento River, inundated floodplains (including the Sutter and Yolo bypasses), and the Delta. Based on observations in Butte Creek and the Sacramento River, young-of-year juveniles typically migrate from November through May. Yearling spring-run Chinook salmon migrate from October to March, with peak migration in November (S.P. Cramer and Associates 1997, Hill and Webber 1999). Downstream migration of yearlings typically coincides with the onset of the winter storm season (Moyle et al. 1989), and migration may continue through March (DFG 1998).
