Type: report

Article abstract: Natural stream hydrology in San Diego County includes both runoff and groundwater inputs. This historically provided for a mixture of perennial and seasonal streams with a wide range of riparian habitats and stream morphologies (Taniguchi and Biggs 2015). Native aquatic species in this region make use of the varied stream habitats with some species requiring permanent or semi-permanent water, such as the southwestern pond turtle (Actinemys pallida, hereafter pond turtle; Rhodin et al. 2021), while others make use of seasonal streams with warmer, shallow, slow-flowing water, such as the arroyo toad (Anaxyrus californicus; Preston et al. 2022). Natural and anthropogenic alterations to streamflow can impact stream morphology and hydroperiods (amount of time surface water is present during the year), which can cause changes to species composition. For example, water diversions and prolonged drought can decrease hydroperiods in permanent streams. Increased runoff from urbanization can increase hydroperiods in seasonal streams (Hawley et al. 2012; Booth and Fischenich 2015). These hydroperiod alterations with associated changes in stream morphology can cause changes in aquatic species compositions with longer hydroperiods facilitating the spread of nonnative aquatic species (White and Greer 2006). The persistence of nonnative aquatic species in previously seasonal streams can have deleterious impacts on native aquatic species, including the arroyo toad (Miller et al. 2012). USGS began to study these relationships between hydroperiod and native aquatic species in San Diego in 2015 by deploying a network of Stream Temperature, Intermittency, and Conductivity data loggers (STICs) on conserved lands throughout the county (Brown et al. 2020b; Brown et al. 2020c). This report presents the results of our stream surveys associated with this network of STIC sites in San Diego County from 2022 to 2024.