The migration of Oncorhynchus mykiss (steelhead trout) from the ocean to upstream spawning grounds is a critical component of the steelhead life-cycle. A depth of 0.6 ft represents the minimum depth of water required for adult steelhead passage, based on work by Thompson (1972). This analysis investigated the discharge required to maintain the minimum depth criteria of 0.6 ft on the lower Santa Clara River (SCR), California from the Pacific Ocean to Piru Creek. Model-based predictions found that a minimum flow of 800 cfs would be required to maintain a 0.6 ft. depth from the SCR estuary to Santa Paula Creek, while 500 cfs is needed to maintain this depth from Santa Paula to Sespe Creek, and 700 cfs would be needed between Sespe Creek and Piru Creek. Results from rainfall-runoff simulations found that the minimum flow criteria were met between Sespe and Santa Paula Creeks for 88-93% of the analysis period and for 96-99% of the time period from Santa Paula to the estuary, when flow in the mainstem near Piru was greater than 400-700 cfs. These results indicate that passage flows are likely to exist throughout the entire mainstem at the same time.

Evaluation of the period 1955-2004 determined that 7-12, 1-day flow events meeting the depth criteria would have occurred over 36% of the flow period from Piru to Sespe Creek. The section of the mainstem SCR from Sespe to Santa Paula Creek had 12 or more 1-day flow events meeting the depth criteria for 68% of the analysis period, and from Santa Paula Creek to the estuary there were 12 or more 1-day flow events meeting the depth criteria for 45% of the time. A second analysis investigated the frequency of flow events with 0.6 ft. depth for a continuous 3-day period. Results found that the reach from Piru to Sespe experienced 7-12, 3-day flow events meeting the depth criteria 14% of the flow period, from Sespe to Santa Paula Creek there would have been 7-12, 3-day flow events meeting the depth criteria for 40% of the analysis period, and from Santa Paula Creek to the estuary 7-12, 3-day flow events meeting the depth criteria have occurred for 26% of the water years analyzed.

The number of potential passage opportunities per year was found to be highly correlated with the total annual runoff, based on a simple regression model. During average and above average water years there appear to be multiple opportunities for fish passage, while during dry years fewer opportunities exist.