Dynamics and Stability of the Hyporheic Environment in Steep Headwater Streams in Western Japan

The hyporheic zone is maintained by the downwelling of stream water into the undelaying sediments, and the environment of the hyporheic zone is dynamic, for example its extent changes seasonally and during high flow events. In this study, we continuously observed the hydrological and biogeochemical environment of the underlying sediments during high flow events to examine the dynamic nature of the hyporheic zone. Two study reaches were established where a series of steps was present in steep headwater streams. The two reaches had the same overall channel gradient of 0.09 m/m, but the number and sizes of steps in the reach were different. The vertical head gradients created by the steps were stable during precipitation events, despite the increase in lateral inflow. Contrast to hydrological environment, the biogeochemical environment fluctuated. Organic matters and suspended sediment concentrations of stream water increased during high flow, and, as a result, the nitrate, organic matter and fine sediment concentration in the downwelling zone of the hyporheic zone increased temporarily. The results of this study suggested that the hyporheic zone of a steep headwater streams may be hydrologically stable, however, in the downwelling zone, where the effects of stream water is strong, the biogeochemical environment is dynamic.