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Hyporheic exchange processes in a coastal headwater stream Scordo, Elisa Branson

Abstract

Hyporheic exchange flow involves the two-way movement of water between the stream channel and the bed and banks. These exchange flows create distinctive habitats and influence biogeochemical processes and water temperatures. This study focused on the characterization of the spatial distribution of subsurface flow pathways and associated travel times through the hyporheic zone within a low-order, high-gradient headwater stream located in the UBC Malcolm Knapp Research Forest, approximately 60 km east of Vancouver, British Columbia. Hyporheic zone processes were examined May to October 2006, at three spatial scales in East Creek: point, channel-unit and reach. Hydrometric data collected from piezometers installed within the stream channel, along with solute injection tracer experiments, were used to characterize subsurface flow pathways within a 100 m stream section. Stream tracer breakthrough curves were used to model the processes of advection, dispersion, lateral inflow and transient storage within the hyporheic zone using the numerical model OTIS-P. Tracer injections at individual step-pool units were used to identify locations of hyporheic discharge, as well as to estimate separate travel times for hyporheic and surface-water transient storage zones. Solute transport process varied with discharge at the reach scale. Transient storage area (As) increased with discharge, while transient exchange coefficient (α) remained fairly constant. At the scale of individual pools, transient storage area and residence times were higher than the reach scale estimate, suggesting that pools and back eddies do contribute to transient storage in headwater streams. Water fluxes calculated with Darcy's Law in one channel-unit did not "scale-up" to the reach scale estimate of hyporheic exchange (α), and was two orders smaller than the reach scale. Direct measurements of water fluxes into the streambed, including vertical hydraulic gradients and infiltration rates, did not vary systematically with discharge. Hydraulic gradients varied significantly with scaled location within the channel-unit, but not with the downstream step height. Hydraulic conductivity varied with site conditions (upwelling, downwelling and neutral sites), suggesting that channel geometry and hydraulic conductivity control exchange flow. This multiple scale approach highlights the considerable spatial and temporal variability and complexity of hyporheic exchange processes within step-pool streams.

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