not anadromous; and there is a great deal of variation in their life histories (Williams, 2006).
When adult salmon, steelhead, and sturgeon return from the ocean and begin their upriver migration, they experience several challenges, including physical and water-quality blockages. Here the delta water system has had a great impact on populations, for 80 percent of the historical spawning habitat for Chinook salmon (Clark, 1929) and much of it for the other species has been blocked by the storage reservoirs of the Central Valley (Lindley et al., 2006). Summer temperatures in the Central Valley waterways can reach potentially lethal levels for salmon, increasing their susceptibility to disease and decreasing metabolic efficiency (Myrick and Cech, 2001, 2004). The timing of adult salmon runs leads them to avoid most of the detrimental effects of high summer temperatures because they enter the delta and swim upriver to their spawning habitats and hatcheries in the spring, autumn, and winter. Wild spawning fish excavate redds in stream reaches with loose gravel in shallow riffles or along the margins of deeper runs (NMFS, 2009), where temperatures are cooler and eggs buried in the gravel receive a sufficient flux of oxygenated water through interstitial flow. The eggs incubate for several months and after emerging the young fry either immediately begin their migration back to the ocean or spend several weeks to a year in freshwater before migrating. Because of this diversity, juvenile salmon and steelhead pass through the delta throughout the year; however, the timing and size of the migrants generally corresponds to specific runs (Lindley et al., 2006; Williams, 2006).
Salmon and steelhead undergo a complex set of physiological changes in preparation for their migration to the ocean known as “smoltification,” after which the young fish are known as “smolts.” The alteration of the fish’s physiology to successfully osmoregulate in saltwater after beginning life in freshwater is a significant challenge that can be exacerbated by human-caused environmental changes (e.g., NRC, 2004b). Most Central Valley Chinook salmon migrate to the ocean within a few months of hatching and the smolts are less than 10 cm long, although some remain in freshwater for up to a year. Juvenile steelhead migrate to sea after one to three years in freshwater, and can be as large as 25 cm in length. Young migrating Chinook are much more vulnerable to entrainment in adverse flows than the stronger-swimming steelhead smolts.
Juvenile salmon migrants experience predation during their downstream migration through the Sacramento River or through the interior delta on their way to the sea. Fish that enter the central delta, driven by the strong tidal and pumping-induced flows, are moved through a labyrinth of channels, which further delays their migration and exposes them to additional predators (Perry et al., 2010). Finally, fish that enter the Old and Middle Rivers (OMR) can be drawn