Localized effects of hyporheic exchange on surface water temperature are well documented – especially in small streams and lateral habitats of larger rivers (e.g., spring channels and back waters). Yet, empirical studies of hyporheic effects on main-channel temperatures over long (e.g, 10’s of km) sections of mainstem rivers are lacking and attempts to model such dynamics have yielded conflicting results. Using data from 57 temperature loggers deployed among 2 intensive study sites, and an additional 22 loggers distributed along 33 km of the Umatilla River, Oregon, USA, we document a cumulative, whole-river temperature effect of hyporheic exchange and show that the magnitude of the effect is correlated with channel pattern (sinuosity). Summertime diel temperature range in the main channel was as low as 4.5°C where channel patterns were complex and associated hyporheic exchange rates were high, but increased to as much as 9°C where the channel was dredged and straightened for flood control, reducing hyporheic exchange. Our results support the hypothesis that channel engineering (e.g., channel simplification for flood control and navigation) has altered historic temperature regimes in gravel-bedded rivers. Restoration of hydro-geomorphic processes that yield complex channel patterns (e.g., channel migration and avulsion) may help re-establish normative main-channel temperature regimes.