Effects of Irrigation Diversion Structures on Chinook Salmon Spawning Migration

Irrigation diversion structures in headwater streams can limit access to spawning habitat and, consequently, reduce individual fitness and population productivity. Environmental and biological factors can also influence the effects of these structures on salmon. Using radio telemetry techniques, we evaluated spring Chinook salmon spawning migration at diversion structures in the Lostine River (northeastern Oregon) from 2008 through 2014. Median passage duration at each monitored diversion (all years combined) ranged from 13 to 104 minutes, whereas median passage duration at a reference site with no structure was 10 minutes. Passage success at structures ranged from 78 to 100%, and 13% of successful passages required multiple attempts. In comparison, passage success at the reference site was 100% and required only single attempts. During this study, a diversion structure located downstream of critical spawning habitat was rehabilitated into a roughened channel, and median passage duration at this site decreased from 77 to 32 minutes. Explicit relationships between environmental conditions (stream temperature and discharge) and fish passage were inconsistent, but the ability to quantify these relationships was hindered by the relatively small range in observed environmental conditions. Overall, these empirical movement data provide a foundation for identifying problem areas and prioritizing and evaluating restoration.