Coarse bed load transport in an alluvial gravel bed stream, Dupuyer Creek, Montana

Coarse bed load was sampled in a gravel/cobble bed stream during two major floods in the snowmelt runoff season. The channel is characterized by high rates of bank erosion and, therefore, high rates of sediment supply and bed load flux. Peak discharge reached four times bank-full, and bed load was sampled at flows 0.7-1.7 times bank-full. A large aperture bed load sampler (1 m by 0.45 m) captured the largest particles in motion, and specifically targeted the coarse bed load size distribution by using a relatively large mesh (32 mm or D-25 of streambed surface size distribution). Bed load flux was highly variable, with a peak value of 0.85 kg/s/m for the coarse fraction above 38 mm. Bed load size distribution and maximum particle size was related to flow strength. Entrainment was size selective for particles D-70. and larger (88-155 mm), while particles in the range D-30-D-70 (35-88 mm) ceased to move at essentially the same flow. Bed load flux was size selective in that coarse fractions of the streambed surface were under-represented in or absent from the bed load. Painted tracer particles revealed that the streambed surface in the riffles could remain stable even during high rates of bed load transport. These observations suggest that a large proportion of bed load sediments was sourced from outside the riffles. Repeat surveys confirmed major scour and fill in pools (up to 0.75 m), and bank erosion (>2 m), which together contributed large volumes of sediment to the bed load. Copyright (c) 2007 John Wiley & Sons, Ltd.