Transient streambed hydraulic conductivity in channel and bar environments, Loup River, Nebraska

Streambed hydraulic conductivity (K) and vertical K (K v) are key controls on groundwater and surface water exchange and biogeochemical fluxes through the hyporheic zone, but drivers of transient hydraulic properties in different fluvial environments are poorly understood. This study combines hydrog...

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Bibliographic Details
Published in:	Hydrological processes 2020-07, Vol.34 (14), p.3061-3077
Main Authors:	Korus, Jesse T., Fraundorfer, Wilhelm P., Gilmore, Troy E., Karnik, Kelsey
Format:	Article
Language:	eng
Subjects:	Aerial photographs Aerial photography Bars (landforms) Bed load Biogeochemistry Electrical conductivity Electrical resistivity Flow characteristics Fluvial sedimentation Fluvial sediments Fluxes Geology Geophysical surveys Geophysics Groundwater Groundwater table Heterogeneity Hydraulic conductivity Hydraulic properties Hydraulic tests Hydraulics Hydrogeology Hyporheic zone Hyporheic zones Patchiness Porosity Radar River channels Rivers Sediment Sediment dynamics Sediment transport Sedimentology Sediments Spatial heterogeneity Stream discharge Stream flow Streambeds Surface water Surface-groundwater relations Temporal variability Temporal variations Tests Water exchange Water table
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Summary:	Streambed hydraulic conductivity (K) and vertical K (K v) are key controls on groundwater and surface water exchange and biogeochemical fluxes through the hyporheic zone, but drivers of transient hydraulic properties in different fluvial environments are poorly understood. This study combines hydrogeology, geophysics, and sedimentology to reveal mechanisms of K and K v transience in the upper 0.5 m of a sandy streambed during low discharge. Hydraulic tests (44 slug tests, 130 falling‐head permeameter tests) and 130 grain‐size analyses were repeated three times over 8 weeks on a 1,200 m2 grid spanning: (a) a channel with continuously flowing water and mobile bed load; (b) an adjacent mid‐channel bar that was stationary and infrequently submerged. Aerial photographs and ground‐penetrating radar show scour and complete reworking of fluvial sediments in the channel. Bar sediments below the water table remained immobile, but infrequent flows of moderate discharge reworked the uppermost few centimetres of the bar top. Despite differences in sediment mobility and stream flow characteristics across environments, K and K v exhibited order‐of‐magnitude differences in spatial heterogeneity and temporal variability in both the channel and bar. Mean K and K v values in the channel were comparatively stable over time. In the immobile bar, mean K declined 20% and K v declined 26% after increased discharge temporarily inundated the bar. Grain‐size distributions were steady across both environments over time, but repeat geophysical surveys of the bar show a decrease in electrical conductivity, likely from porosity reduction. These findings suggest that sediment dynamics and stream flow characteristics in different streambed environments are important drivers of K transience during low discharge conditions. Specifically, pore clogging can be an important mechanism of transience over short durations (weeks to months) in immobile sediments subject to infrequent flows and minor reworking. In the sandy Loup River, sediment dynamics, pore clogging, and stream flow characteristics in different streambed environments are important drivers of K transience during seasonal low‐flow conditions.
ISSN:	0885-6087 1099-1085