Gravel-sand transition in the mixed bedrock-alluvial Congaree River, SC. Preliminary results on gravel mobility 

Abstract

<p>Gravel-sand transitions are generally characterized by a decline in bed slope and surface grain size from about 10 mm to 1 mm over a range of distances. Values vary from a few hundred meters on relatively small streams in Scotland and Canada, to tens of kilometers on the Fly River in Papua New Guinea and on the Fraser River in Canada. Gravel-sand transitions form for different reasons such as preferential breakdown of sediment clasts, downstream propagation of a gravel front, changes in sand transport mode and different mobility between sand and gravel. Recent field work on the Congaree River, South Carolina, revealed the intermittent presence of gravel bars and sand bars, as well as bedrock channel reaches along the upper 35 km. At ~30 km downstream of the reach where gravel is observed, the river channel and the bars are made of fairly uniform sand with median grain size of 0.6 mm. It is thus reasonable to expect the presence of a gradual gravel-sand transition. Field measurements of bed elevation show that this gravel sand transition is not associated with significant bed slope change. We hypothesize that the presence of the gradual transition is due to the increased gravel transport capacity in the presence of sand. Further, the resistant bedrock in the mixed bedrock-alluvium reach may prevent the abrupt change in bed slope observed in other gravel-sand transitions.</p>