THE INTERDEPENDENCE OF DIEL TEMPERATURE CYCLES IN SURFACE AND HYPORHEIC WATER ON THE UMATILLA RIVER FLOODPLAIN, OREGON

The interdependence of surface and hyporheic temperature dynamics is well documented in small streams and lateral habitats of larger rivers (e.g., spring channels and back waters). Yet, few empirical studies have investigated hyporheic effects on main-channel temperatures over long (e.g, 10’s of km) sections of mainstem rivers. Attempts to model such dynamics have yielded conflicting results. In the Umatilla River, Oregon, USA, data from 57 temperature loggers deployed among 2 intensive study sites, and an additional 22 loggers distributed along 33 km of the river, revealed a cumulative influence of hyporheic exchange on whole-river diel temperature patterns. The magnitude of the effect was correlated with channel pattern (sinuosity). Where channel patterns were complex, high hyporheic exchange rates yielded a main channel diel temperature range as low as 4.5°C. In contrast, dredging and straightening the channel for flood control doubled the diel temperature range to 9°C. Our findings suggest that past channel simplification for river navigation and flood control has altered diel temperature patterns in channels and hyporheic zones, thereby changing the physical context within which biogeochemical processes occur in rivers and across floodplains.