Pool-Riffle Adjustment Due to Changes in Flow and Sediment Supply

How do gravel bed pool-riffle streams adjust to changing upstream water and bedload sediment supplies, and what analysis techniques can help to effectively identify how change occurs? Here, we use a mixture of field and experimental data to examine these problems and apply a suite of traditional and novel analysis approaches to highlight dynamics which might otherwise go undetected. Eleven years of monitoring channel morphology in a small forested watershed indicate that pool-riffles persist through large changes in upstream water and bedload supply and that bed architecture relief is correlated to flow magnitude. A flume experiment consisting of eight runs was conducted to examine the field case in more detail. The experimental design splits the eight runs into four runs of relatively high water and sediment supply and four of relatively low water supply, with no upstream sediment supply. Experimental results corroborate the field-based measurements of pool-riffle persistence, which is due to a coupling between downstream width variations, and spatial patterns of flow velocity and bedload transport. More specifically, measurements made during the flume experiments along a prominent pool-riffle pair indicate that temporal and spatial changes to topography, flow hydraulics, and bed surface sediment texture are more rich and nuanced than existing generalizations offer. For example, clustering analysis completed using self-organizing maps indicates that sediment sorting between pools and riffles is not simply a binary type response of finer versus coarser described by some characteristic grain size.

Automated summary

This study investigates how gravel bed pool-riffle streams adjust to changing water and sediment supplies using a combination of field and experimental data. The research highlights that pool-riffles persist through large changes in water and sediment supply, with bed architecture relief correlated to flow magnitude. Experimental results corroborate field-based measurements, showing a coupling between downstream width variations and flow velocity patterns. Measurements also reveal that sediment sorting between pools and riffles is more nuanced than previously thought.