Fall 2012

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)


Science & Environmental Policy


Anadromous steelhead (Oncorhynchus mykiss) populations on the central coast of California have been reduced to critical levels throughout the last century. Six streams run through the Coast Dairies Property near Davenport, CA; three of the streams are known to support steelhead, and three do not support any known populations. This study examined two physical factors that are potentially limiting steelhead success in the three small streams. The first potential limiting factor examined is low density substrate found in the three streams (Santa Cruz mudstone). The presence of low density substrate could lead to increased risk of redd ‘washout,’ or destruction of the steelhead nest and the associated eggs due to gravel mobility. Alternatively, the presence of low density substrate could shift the size range suitable for spawning. This study used tracer stones of mudstone and granite to populate a logistic regression model that can be used to predict the probability of entrainment for a given particle under defined shear stress conditions. The second potential limiting factor is a migration barrier caused by the presence of a culvert on each stream where it passes under Highway 1. Culverts in the three streams without steelhead and two culverts on streams that have known populations of steelhead were surveyed and modeled using a 1-dimentional hydraulic modeling program. Three separate passage criteria were used to assess if the modeled hydraulic conditions were suitable for steelhead passage.

We found that both factors were potentially limiting steelhead success in the three streams. This study shows substantial evidence that the low density mudstone substrate is more mobile than typical granitic particles. The results indicate a mudstone particle has a probability of motion approximately 30% higher than a similar granite particle under the same flow conditions. This increase in mobility could result in increased redd washout frequency, and might limit steelhead spawning success. The results of the culvert modeling study revealed it is highly likely the tunnels are presenting a barrier at most flows. The two study culverts what are known to pass steelhead had noticeably better depth and velocity conditions than the three culverts with unknown passage suitability. One of the three passage criteria, suggested by the National Marine Fisheries Service and the California Department of Fish and Game, indicated none of the five culverts were passable, suggesting that it may be too conservative to reflect the swimming abilities of steelhead on the central coast of California.