Sediment-transporting flows in headwater streams

The equilibrium alluvial stream channel has a geometry that allows it to pass the water and sediment supplied from the watershed. At the same time, the equilibrium alluvial channel is built and maintained by the flows and sediment delivered to it. A prerequisite for understanding the creation of the equilibrium channel is an understanding of the sediment conveyance and competence of the flows the channel receives. This study describes the bed-load transport regime as it is linked to hydrology and geomorphology in 23 headwater gravel-bed streams in snowmelt-dominated parts of central and northern Idaho. At sites, drainage areas range from 1.29 to 381 km 2 , stream gradients range from 0.0042 to 0.0747, and median bed surface particle sizes range from 4 to 207 mm. Stream architecture includes riffle-pool, planar, and step-pool beds. The bed load is much finer than the surface and subsurface material, suggesting selective transport of the finer sizes. Nonetheless, the majority of the load is sand at all flow discharges. Progressively coarser sediment was collected as flow discharge increased, and painted rock experiments documented the transport of coarser particles at higher discharges. The supply of sediment to the streams appears limited, as indicated by observed clockwise hysteresis in bed-load transport rates during each spring snowmelt and by the coarse surface armor observed at sites. Flows above bankfull discharge move 37% of the bed load, whereas flows between mean annual discharge and bankfull move 57% of the bed load. The bed-load effective discharge has a recurrence interval that averages 1.4 yr and the magnitude of effective discharge averages 80% of bankfull discharge. The recurrence interval of bankfull discharge averages 2.0 yr. The ratio of effective discharge to bankfull discharge is independent of basin size, grain size, and gradient, although the ratio increases with the relative magnitude of large infrequent events.

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