Species that breed in stream environments with unpredictable interannual variability in hydrological regimes may exhibit plasticity in the timing of their breeding activities. Breeding phenology should coincide with conditions and habitats that maximize a species' reproductive success. For the lotic-breeding frog Rana boylii, the timing of breeding activities of a population can vary by more than a month among years. To examine the influence of abiotic factors on the variation in the timing of onset and patterns (i.e., peaks and pauses) in breeding activity of R. boylii, we sampled seven geographically separated sites that covered an extensive portion of the species' range in northwestern California for two breeding seasons. We collected daily environmental and male vocalization data and conducted weekly egg mass surveys at breeding sites. Here, we found the timing of calling activity and oviposition varied markedly among geographically separated sites and between years. Water depth and water temperature influenced calling phenology, whereas water depth and both temperatures (water and air) were significant factors in the timing of oviposition. In general, breeding activity did not commence until water temperatures reached 10°C. Calling and oviposition occurred later at deeper sites with colder, spring water temperatures. Models that predict the timing of breeding activities can improve survey and monitoring efforts and can assist managers of regulated streams in developing flow assessments that are compatible with species' breeding requirements. This information may be particularly useful in developing individual based models to assess overall reproductive success.