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All California Chinook salmon are similar in appearance, with some slight differences in size, shape, and coloration. Central Valley spring-run Chinook salmon are separated by their run timing, spawning areas, and late maturation in fresh water.

19th-century spring-run Chinook salmon were once as abundant as fall-run Chinook in the Central Valley, numbering approximately one million returning adults per year. Over the past forty years, annual abundance has varied considerably, from highs around 30,000 fish to lows of about 3,000 fish. Since 2012, population estimates have plummeted.

Spring-run Chinook salmon migrate upstream during high runoff events starting in January or February. High flows, especially from snowmelt, allow adults to access higher elevation, smaller tributaries in April through June that are generally inaccessible to salmon at other times of the year. Adults seek out deep, cool pools in tributary streams less than 21°C (70°F) where these big fish hold over the summer before spawning in the fall. They prefer pools with plenty of cover, such as rock ledges, bubble curtains, and woody debris. Most spring-run Chinook adults in the Central Valley are four years old and average 78.5cm (31 in.). Juvenile spring-run Chinook spend varying amounts of time in freshwater before migrating to sea: 1) a matter of weeks after hatching, 2) a few months after hatching, or 3) an entire year or more in fresh water.

The delayed maturation strategy in spring-run salmon arose as a single evolutionary event in a Chinook population and spread throughout the species’ range, primarily through straying of fish to other watersheds. There are only two distinct self-sustaining populations of Central Valley spring-run Chinook salmon: one in Deer and Mill creeks and one in Butte Creek.