Groundwater Sources and their Influence on the Distribution of Steelhead in Topanga Creek, Topanga, California

Abstract

Abstract This study examined the effects of groundwater on the distribution of steelhead (Oncorhynchus mykiss) in Topanga Creek, a small coastal stream in Los Angeles County, California. Water quality was monitored in Topanga Creek and in surface discharge from 20 springs on a monthly basis from April to December 2005. Instream habitat mapping and monthly snorkel surveys were also conducted to determine whether steelhead preferentially selected habitat units with groundwater. Sixteen percent of habitat units in the Topanga Creek study area received groundwater from known springs. For six out of the nine months of the study, significantly more trout were found in groundwater areas than would be expected by the frequency of such sites. The most likely reason for steelhead to select habitat units with groundwater during this study was differences in habitat between areas with and without groundwater. Habitat units with groundwater had characteristics that were more favorable for steelhead such as greater surface area, greater depth, and higher shelter value. Habitat units with groundwater may also have acted as refugia from high stream temperatures. While there was no significant difference between temperature of the creek and temperature of surface discharge from springs, deep pools were significantly cooler than shallow pools. Water quality in both the creek and springs was sufficiently good for trout throughout the study. These findings suggest that groundwater may play an important role in maintaining steelhead populations near the southern extent of their Pacific coastal range