STREAM GEOMORPHOLOGY, WATER SURFACE SLOPE, AND IMPLICATIONS FOR PATTERNS IN HYPORHEIC EXCHANGE

Pool-step and pool-riffle sequences drive patterns of upwelling and downwelling through bed sediments in mountain streams. In this study we investigate how streambed features affect hyporheic exchange flow across a range of scales. Survey equipment and measuring tapes were used to create longitudinal profiles for 12 randomly selected second-, third-, and fourth-order stream reaches in the Lookout Creek basin in Oregon. Stream longitudinal profiles were broken into segments based on slope breaks, and segments were assigned to one of three categories: flat water (slope less than 0.01), steep water (slope between 0.01 and 0.13), or step (slope greater than 0.13). Regression models showed that segment length, excluding steps, increases predictably with increasing watershed area after accounting for the effect of segment slope (two-sided p-value < .0001 for the overall significance of the regression). We hypothesize that patterns of downwelling and upwelling are predictable from water surface slope breaks, and that the longitudinal lengths of upwelling and downwelling areas will increase with increasing watershed area. We installed piezometers at regularly spaced intervals in a sub-sample of stream reaches to measure vertical hydraulic gradients and characterize patterns of upwelling and downwelling. Preliminary results show that short upwelling areas occur where water surface slope breaks are concave-up, and that the longitudinal length of downwelling areas increases with increasing distance between slope breaks. These results suggest that the pattern of upwelling and downwelling in these streams may be predictable, and scaled according to the size of streambed features.