Fragmentation of an Intermittent Stream During Seasonal Drought: Intra‐annual and Interannual Patterns and Biological Consequences

Intermittent streams lose surface flow during some portion of the year and can be important breeding and rearing habitats for stream biota. However, habitat contraction and deteriorating water quality across the summer can result in harsh conditions and mortality. We explored patterns of drying in a small intermittent stream across the summer in Mediterranean‐climate California, including across 4 years that differed in antecedent precipitation. Wet–dry mapping revealed earlier stream fragmentation following dry winters and that entire sections of the stream varied in their propensity to dry suggesting an important influence of geomorphology on drying. Within two ‘slow‐drying’ reaches, initial riffle volumes were higher following wetter winters, but the rate of riffle drying was higher following wet years, presumably because higher initial volumes resulted in greater drying capacity. Initial pool volumes were similar across years, but the rate of pool drying was faster following dry versus wet winters (pool half‐life ranged from 9.7 weeks in the driest year to 26.3 weeks in the wettest year). Stream temperature differed among years, but differences were slight, and temperatures rarely exceeded optimal conditions for trout growth. We observed limited movement of trout during drier years and found that movement was negatively associated with pool depth, riffle length and date, and positively associated with riffle volume. Overall, we found that antecedent rainfall influenced variability in pool drying more than riffle drying, that entire sections of the creek varied in their propensity to dry and that biological fragmentation preceded physical fragmentation by 3 to 7 weeks. Copyright © 2015 John Wiley & Sons, Ltd.