ABSTRACT: -Stream, hyporheic, and hillslope flow interactions along a headwater channel.

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Abstract

Hyporheic exchange is an important control on solute transport and transformations in riparian-stream corridors. Dynamics of hyporheic flow vary with stream discharge, substrate type, channel morphology, and the hydrogeomorphology of the hillslope-riparian-stream continuum. Because of the tight linkages between the hillslope and channel, and the presence of geomorphic features that promote hyporheic exchange (e.g., channel steps), interactions between stream, hyporheic, and lateral hillslope flows in a forested headwater channel may be unique compared to its downstream counterpart. We examined interactions between streamwater, hyporheic exchange, and lateral inflow using constant rate injection of a conservative solute during low and high base flow periods in three reaches (upper, transitional, and lower) along a headwater channel in coastal British Columbia. Piezometers installed in the stream bed and riparian zone were used to measure hydraulic gradients and sample tracer concentrations in the subsurface. Transient storage and lateral inflow were estimated using the OTIS-P computer code. Lateral inflow from the hillslopes appeared to occur at discrete locations rather than as dispersed input for all reaches. In the upper reach (mean channel gradient 17%, bankfull width 1.0 m), very little hillslope contribution to base flow was observed, even though the channel was confined by the hillslope. The greatest lateral hillslope flow occurred within the transitional reach (mean channel gradient 11%, bankfull width 1.5m), which was located at a topographic break between the steep upper and gentler lower reaches. In the lower reach (mean channel gradient 5%, bankfull width 1.3m), which had a wider and flatter riparian area, hillslope inflow was intermediate between those of the upper and transitional reaches.For all reaches, greater hyporheic exchange was observed during low base flow than high base flow conditions. In sections dominated by steps formed by woody debris and boulders, we observed strong downward hydraulic gradients upstream of the steps, and weak upward gradients in the downstream pools. Significant tracer infiltration occurred in the sediment wedges upstream of wood steps, consistent with the downwelling component of a hyporheic flow path. Hyporheic flow and subsurface mixing were subdued in the vicinity of concentrated lateral discharge points of hillslope flows. The pools appeared to be favored locations for lateral inflow from the hillslopes. The interactions among stream, hyporheic and hillslope flows appear to be controlled dominantly by the channel steps, which produce vertical exchange across the bed, as well as by the gradients of adjacent hillslopes and the width and gradient of the riparian zone, which influence both the lateral inflow of hillslope flow and possibly the opportunity for lateral hyporheic flows.

Bibliography of Canadian Geomorphology