Rapids in bedrock rivers: morphological characteristic, distribution, and flow dynamics

Rapids are a channel morphology common to bedrock-bound rivers that have been primarily identified by their characteristic chaotic flow. These complex flow structures develop due to reductions in channel cross sectional area through vertical steps in the bed or lateral constrictions. Flow features that develop include a tongue of convergent accelerated flow, followed by standing or breaking waves downstream, and horizontal recirculation eddies that may form sediment deposits where space permits. Rapids enforced by debris fans encroaching on the channel are relatively well studied in major canyons of the American Southwest, but rapids also occur in bedrock-bound channels without debris flow deposits.  

We explore the distribution, controls, and characteristics of rapids in bedrock-bound rivers using satellite imagery and rafting guidebooks. We delve deeper in the dynamics of rapids in the 375 km long Fraser Canyon, British Columbia, Canada where diverse rapid morphologies are observed. Here we use high resolution observations of flow and topography to categorize rapid types and their causes. We find that there are two broad categories of rapids: sediment-controlled and bedrock-controlled. Those rapids can be further divided into two types based on the nature of the channel cross-sectional area reduction: constriction-type and step-type. Sediment-controlled rapids are created and maintained by mass movement processes where rapid characteristics are dependent on particle size, magnitude, frequency of sediment supply, and location of sediment input. Bedrock-controlled rapids have their morphology imposed by the structure of the rock that creates lateral constrictions or bedrock steps. Although we have defined broad categories, many rapids in the Fraser Canyon are the result of mixed controls and types.  

Our observations indicate that rapid dynamics are discharge dependent and changes in discharge affect rapid subcategories differently. As discharge changes, rapids may wash out or become larger and more severe. Preliminary results show that sediment-controlled rapids tend to wash out as discharge increases due to the submergence of boulders and/or overtopping of the debris fan which reduces the constriction ratio. At bedrock-controlled rapids, complex channel geometry results in varied responses to changes in discharge. Rising water levels may lead to both overtopping of obstacles or new obstacles. Bedrock-controlled rapids become more severe where new obstacles are inundated as discharge increases.