Hyporheic flow as a potential geomorphic agent in the evolution of channel morphology in a gravel-bed river

Morphologic and hydrologic properties of head-cut gullies formed in meandering bends of a gravel-bed river in northern Japan were examined to investigate their topographic development. head-cut gullies had incised to 2 m below the floodplain surface and had perennial seepage outflows. Because no surface runoff was observed across the meandering necks, we hypothesized that hyporheic flow erosion cutting into the down-river edge of the meandering necks is one factor for the development of head-cut gullies. Several topographic features caused by sapping and piping erosion were observed within the head-cut gullies. A tracer experiment and examination of the water table, water chemistry, and water table responses in wells on the floodplain and within the head-cut gully revealed that significant preferential hyporheic flow occurred between coarse cobble and fine sediment layers of the floodplain materials. During a storm event, water tables around the head-cut gully quickly responded to changes in the water table of the stream channel; this result also suggests that hyporheic flow occurred across the meandering bends. Hyporheic flow around the head-cut gullies had higher hydraulic conductivities and preferential flow paths that may relate to buried beds of paleochannels. An erosion model suggests that it is unlikely that hyporheic flow alone had enough energy to account for all the soil erosion. Thus, a combination of hyporheic flow (sapping erosion) and channel erosion (bank erosion) appeared to contribute to the formation of the head-cut gullies. Hyporheic flow is an important component in the geomorphic evolution of channels and the floodplain in a gravel-bed river.