1. The emergence time of Pteronarcys californica in streams in the Henry's Fork catchment, Idaho, U.S.A. was negatively correlated with mean April water temperature. Emergence was in mid- to late May at sites influenced by groundwater, where April water temperature averaged 7.9 °C. Adults emerged in mid-June in streams receiving run-off from snowmelt (mean April water temperature 5.4 °C). Intermediate emergence times were observed in a regulated section of river where water temperature was influenced, on one bank, by dam release (mean April water temperature 4.5 °C) and, on the other, by a spring-fed tributary stream (mean April water temperature 6.3 °C).
2. During each of the three study years, emergence was earlier on the bank of the regulated section that was warmer during April and May. The mean body length of P. californica exuviae, collected from the warm side of the river, averaged 1.2 mm longer than those collected from the cold side.
3. We tested the effect on emergence of altering springtime water temperature by translocating P. californica in cages from one location to another during April. Individuals moved to sites with higher April water temperature emerged earlier than individuals that remained at the site from which they were collected.