PREFERENTIAL FLOW-PATHS IN HYPORHEIC OPEN-FRAMEWORK GRAVELS IN BRAIDED RIVER SEDIMENTS

Preserved networks of interconnected, clast-supported, large diameter pore spaces that provide conduits for rapid water transport are found within near-horizontal beds of coarse open-framework sediments naturally deposited and modified by braided rivers. These pore networks provide highly conductive pathways that dominate water flux in fluvial bars and bar complexes and become interconnected at scales of 10's to 100's of meters. They are here hypothesized to provide rapid flow through extensive hyporheic exchange networks in the braided river-flood plain setting. However, these thin open-framework gravel beds have not been tested by other workers to determine their importance in the hyporheic flow scheme.

At two field sites in the Nyack floodplain in Montana, coarse, open-framework layers have been geologically located within braided river bar sediments, duplicating what others have found in other braided river settings. To hydrologically detect these coarse-grained layers, a number of small vertical-interval techniques were used to detect ground water velocities and hydraulic conductivities developed in closely-spaced wells along a flow-line. Geophysical techniques, including GPR and electrical resistivity tomography, were used to detect braided bar architecture in comparison with the geologic and hydrologic findings, and to extend the geology of the sediments beyond the wells. The demonstrated comparison of the geologic, geophysical and hydrologic results allows the detection of the link between sedimentological conditions and hyporheic exchange mechanisms in the braided river setting.