Geomorphic controls on spatial distributions of cobbles and boulders in stream-channel networks

Abstract

Cobbles and boulders (CoBo) in stream channels provide overwintering habitat for juvenile steelhead trout (Oncorhynchus mykiss). Lack of overwintering habitat may be a key limiting factor causing dwindling steelhead populations. Here, I develop a model for predicting the spatial distribution of CoBo in Pescadero Creek, San Mateo County, northern California, which could be used to estimate the watershed-scale extent of overwintering habitat. The model is based on the theoretical expectations that (1) bed sediment is at threshold of motion at bankfull discharge, (2) debris flows deliver large material to predictable locations in stream channel networks, (3) shallow landsliding is a primary source for debris flows, and (4) that durable source bedrock is necessary for CoBo to occur. I develop and test the model with a combination of field reconnaissance, laboratory rock strength testing, and GIS analysis of the channel network and upstream shallow landsliding potential and rock type. The model correctly predicts the occurrence or lack of CoBo at 90% of the field-verified sites using criteria for channel slope, drainage area, and upstream extent of either hillslope instability or durable bedrock. From measurements of refugia density in two reaches, I estimate that CoBo can support 0.5 fish/m2 for juvenile steelhead.