CAN GRAVEL AUGMENTATION COOL A LARGE, GRAVEL-BED RIVER?

The hyporheic zone serves as transient storage within a river channel, where solutes and heat in river water are retained for periods of time before being released back into the river. The residence time of water within the hyporheic zone can lead to a temporal phase shift between hyporheic and mainstem temperature, where cooler hyporheic water emerges back into a warmer mainstem and vice versa. Several studies have documented this buffering effect influencing overall river temperature in smaller, headwater streams, but little data exist for larger rivers. This study investigates the sensitivity of a regulated, gravel bed river to the temperature buffering effects of hyporheic exchange and postulates how gravel augmentation could cool a river by increasing hyporheic exchange.

Hyporheic exchange was primarily identified on a 24-km stretch of the 6th-order Clackamas River in northwestern Oregon by temperature anomalies, which are patches of water that demonstrate at least a 1 deg C temperature difference from the mainstem. Forty hyporheic temperature anomalies were identified through field investigations and thermal-infrared radiometry (TIR) in summer 2006. The location of the anomalies was intimately connected with geomorphic features on gravel bars (bar channels and heads) that focus flow along preferential pathways. Higher hyporheic discharges emerged from anomalies where bars demonstrated high hydraulic conductivity, which occurs in gravel that has been recently reworked. Overall, the river cooling effect is small, as hyporheic anomoly discharges consitute only a fraction of mainstem discharge.

However, adding gravel to a river will form new bars and/or enhance older bars, which will increase the amount of hyporheic exchange occurring throughout the river. This can lead to the development of new temperature anomalies that may serve as localized fish refugia when mainstem temperatures are too warm.