20-Years of Hyporheic Research at the H. J. Andrews Experimental Forest

The hyporheic zone is the area below the streambed and in the unconfined aquifer adjacent to the stream where stream water is found in the subsurface. Hyporheic exchange flow is the movement of stream water from the surface channel into the subsurface and back to the stream over relatively short periods of time. We have been studying the hyporheic zone in the mountain stream network of Lookout Creek at the H. J. Andrews Experimental Forest and LTER for more than 20 years. This poster will summarize the results of that research. Our work has shown that a variety of channel morphologic features drive hyporheic exchange, but changes in longitudinal gradients over pool-step or pool-riffle sequences account for most of the exchange flow in any given stream reach. We have used groundwater flow models to examine the location, length, and residence times of hyporheic flow paths, and using this knowledge of residence times, we have shown how residence time is related to the type and magnitude of effect that hyporheic exchange flows can have on stream temperatures and stream nitrogen cycles. Finally, using empirical data from several modeling studies, we have developed a regression relationship between stream discharge and hyporheic exchange useful for estimating of the amount of hyporheic exchange flows that could be expected in mountain streams of different sizes.