Unit-bar migration and bar-trough deposition: impacts on hydraulic conductivity and grain size heterogeneity in a sandy streambed

The hydrologic function of riverbeds is greatly dependent upon the spatiotemporal distribution of hydraulic conductivity and grain size. Vertical hydraulic conductivity (K_v) is highly variable in space and time, and controls the rate of stream–aquifer interaction. Links between sedimentary processes, deposits, and K_v heterogeneity have not been well established from field studies. Unit bars are building blocks of fluvial deposits and are key to understanding controls on heterogeneity. This study links unit bar migration to K_v and grain size variability in a sand-dominated, low-sinuosity stream in Nebraska (USA) during a single 10-day hydrologic event. An incipient bar formed parallel to the thalweg and was highly permeable and homogenous. During high flow, this bar was submerged under 10–20 cm of water and migrated ~ 100 m downstream and toward the channel margin, where it became markedly heterogeneous. Low-K_v zones formed in the subsequent heterogeneous bar downstream of the original 15–40-cm-thick bar front and past abandoned bridge pilings. These low-K_v zones correspond to a discontinuous 1-cm layer of fine sand and silt deposited in the bar trough. Findings show that K_v heterogeneity relates chiefly to the deposition of suspended materials in low-velocity zones downstream of the bar and obstructions, and to their subsequent burial by migration of the bar during high flow. Deposition of the unit bar itself, although it emplaced the vast majority of the sediment volume, was secondary to bar-trough deposition as a control on the overall pattern of heterogeneity.