Pool-Riffle Formation in Mountain Streams

Abstract:

Pool-riffle formation in low-sinuosity mountain streams is thought to occur through at least two different mechanisms: (1) a transient flow obstruction converges flow, drives pool development and results in an upstream and downstream riffle, (2) variations in channel or valley width set up converging and expanding zones of flow, which leads to pool and riffle development, respectively. These two mechanisms require a width gradient which is not always present, or obviously so along all pool-riffle channel reaches. We propose a third formative mechanism to account for nearly uniform channel width profiles which depends on a spatially discrete and strong change in the bed surface grain size distribution that is characterized as coarse and perhaps better sorted than prevailing conditions. We are testing each of these formative mechanisms plus a no forcing alternative with physical experimentation and numerical modeling. We will present initial observations and findings from our work, focused on addressing the following three questions: (a) does each formative mechanism result in pool-riffle pair development? (b) if a pool-riffle pair forms, are the associated perturbations to the flow and sediment transport fields sufficient for development of additional pool-riffle pairs? and (c) are resultant morphologies comparable in terms of geometries?