REVISED VELOCITY-REVERSAL AND SEDIMENT-SORTING MODEL FOR A HIGH-GRADIENT, POOL-RIFFLE STREAM

Sediment-sorting processes related to varying channel-bed morphology w ere investigated from April to November 1993 along a 1-km pool-riffle and step-pool reach of North Saint Vrain Creek, a small mountain strea m in the Rocky Mountains of northern Colorado. Measured cross-sectiona l areas of flow were used to suggest higher velocities in pools than i n riffles at high flow. Three hundred and sixteen tracer particles, ra nging in size from 16 mm to 256 mm, were placed in two separate pool-r iffle-pool sequences and used to assess sediment-sorting patterns and sediment-transport competence variations. Tracer-particle depositional evidence indicated higher sediment-transport competence in pools than in riffles at high flow. Pool-riffle sediment sorting may be created by velocity reversals, and more localized sorting results from gravita tional forces along the upstream sloping portion of the channel bed lo cated at the downstream end of pools.