THE SIMULATION OF POOL-RIFFLE MORPHOLOGIES IN NORTHEAST CHANNELS WITH DIFFERENT DENSITIES OF LARGE WOODY DEBRIS AND BOULDERS

Large woody debris (LWD) provides important habitat for aquatic species and is often associated with formation of pools and scour holes. Studies on forced pools include many reports of over 80 or 90 percent of pools being associated with structural controls and large obstructions that include boulders, bedrock outcrops and LWD. A computer model was developed that attempts to predict the percent area covered by pools, riffles, glides and runs based on input data that include channel slope, width, the number of small and large boulders, and the number of small, medium and large pieces of wood. The statistical-empirical model is founded on the idea that boulders, bedrock outcrops and LWD provide a physical framework that then controls local hydraulic conditions and the locations of pools and riffles. The spacing of individual obstructions approximate and are modeled as log-normal distributions with separate distributions for different size and types of obstructions. Pools are assigned different probabilities of development depending on the obstruction type. Subsequent pool and riffle lengths follow their own slope-dependent, log-normal trends. A minimum distance develops between successive pools because of the backwater conditions and turbulent conditions needed for pool formation. The total number and spacing of pools, riffles, glides and runs thus reflects the characteristics of the distribution of obstructions and characteristics of the pool-riffle couplet. Statistical results provide evidence that the number of pools at over 30 field sites is influenced by the number of log jams, the number of large boulders, the number of pieces of wood and channel width. The number of scour holes also showed some statistical relation to channel width, slope, number of pieces of wood, and number of boulders.