Persistence of armor layers in gravel-bed streams

Streambed surfaces are typically coarsened, or armored, at low flows, but there is little evidence of their condition during floods, when significant hydraulic and ecologic disturbance occurs. Some flume experiments have been used to conclude that armor layers wash out during floods, although other experiments have produced a persistent armor layer. In the absence of clear field or flume evidence, we use a surface-based transport model in an inverse prediction of surface grain size as a function of transport rates observed in the field. The predicted surface grain size matches that observed at low flow and indicates that low-flow armor layers persist at large flows. In the field, transport grain size increases with transport rate, reducing or eliminating adjustments in bed surface grain size as flow and transport increase. A persistent armor layer considerably simplifies the prediction of sediment transport, hydraulic roughness, and habitat disturbance during floods.