Abstract
Stream restoration designed specifically to enhance hyporheic processes has seldom been contemplated. To gain experience with hyporheic restoration, an engineered streambed was built using a gravel mixture formulated to mimic natural streambed composition, filling an over-excavated channel to a minimum depth of 90 cm. Specially designed plunge-pool structures, built with subsurface gravel extending down to 2.4 m, promoted greatly enhanced hyporheic circulation, path length, and residence time. Hyporheic process enhancement was verified using intra-gravel temperature mapping to document the distribution and strength of upwelling and downwelling zones, computation of vertical water flux using diurnal streambed temperature patterns, estimation of hyporheic zone cross section using sodium chloride tracer studies, and repeat measurements of streambed sand content to document evolution of the engineered streambed over time. Results showed that vertical water flux in the vicinity of plunge-pool structures was quite large, averaging 89 times the pre-construction rate, and 17 times larger than maximum rates measured in a pristine stream in Idaho. Upwelling and downwelling strengths in the constructed channel were larger and more spatially diverse than in the control. Streambed sand content showed a variety of response over time, indicating that rapid return to an embedded, impermeable state is not occurring.
Highlights
A river’s boundary does not end at the channel margins
Vertical water flux at the Kingfisher site was estimated at 0.019 m/d
The vertical water flux values at Kingfisher after restoration are significantly greater than those reported in pristine streams
Summary
The river system includes physical processes that extend laterally, into the riparian zone and floodplain, and vertically, into the substrate beneath the channel and floodplain [1,2,3] This transitional zone between subterranean and surface aquatic environments, commonly referred to as the hyporheic zone [4], provides ecological functions/benefits that help sustain streambed and aquatic conditions. Among these functions are vertical water flux between the stream and subsurface, water temperature moderation, recycling of carbon, energy, and nutrients, natural attenuation of certain pollutants, a sink/source of sediment for the channel, and habitat for benthic and interstitial organisms [4]. The argument is put forth that, by constructing a channel that mimics a natural alluvial morphology, and by reestablishing natural processes of sediment mobilization, transport, and deposition, the hyporheic functions will be
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