Pools and riffles in the bedrock-alluvial South River, VA are systematically different than pool-riffle sequences in alluvial rivers

Abstract

Understanding the characteristics, formation, and spacing of bedforms in different types of rivers is key to being able to guide channel restoration. Two of the most widely recognized and studied bedforms are pools and riffles, and previous studies which investigate pool-riffle sequence spacing focus primarily on rivers composed of a single substrate. This study will document pools in an environment composed of significant amount of both bedrock and alluvial material (i.e., a mixed bedrock-alluvial river) in order to determine how frequent bedrock outcrops impact bedform morphology and spacing. A variety of geomorphic survey techniques are used to create a longitudinal profile of 4.24 km of the South River, VA, which was analyzed in order to determine pool locations and their characteristics (such as spacing and depth). The South River bed profile was also then compared to a bed profile from a flume study by Lisle et al. (1997), in which a 160 m long 1 m wide flume acted as a setting in which pools and riffles can form in the absence of any controls except for active bedload transport. ☐ Results indicate there are significant quantifiable differences between bedforms in the South River and those identified in the flume, and between forced and non-forced pools in the South River. Of the 24 pools which were identified in the South River data, 13 (or 54%) of these pools are associated with areas of bedrock, areas where the river impacts the valley margins, or Dooms Dam. These forced pools are, on average, substantially deeper than unforced pools. Despite these deep forced pools raising the average of the South River pool data, all pools in the South River are still 44% shallower than pools in the flume. Additionally, removing forced pools from the South River pool data yields a cumulative distribution which is symmetrical and normally distributed, much like the cumulative distribution of the flume data. Pool spacing in the South River was found to fall within the 5-7 channel width range that is cited in numerous other studies, with a mean spacing value of 6.3 channel widths. Pools in the flume were found to be more closely spaced and fall outside this range with a value of 3.6 channel widths. Ultimately, these results indicate that pool spacing is unaffected by bedrock forcing, while pool depths are substantially reduced.