ABSTRACT

Alluvial valleys in southern Arkansas and northern Louisiana commonly do not follow a smoothly graded longitudinal profile. Rather, they generally follow a stepped trajectory with alternating lower- and higher-gradient sections. The remarkable aspect of this valley morphology is that the low- and high-gradient sections occur primarily within distinct east-west zones (for south-flowing streams) and secondarily within distinct north-south zones (for east- and west-flowing streams). The higher or lower gradients are observable in all well-developed stream valleys that cross a zone, regardless of the natural valley gradient. In the lowest-gradient valleys, low-gradient sections often have a reversed slope, with the alluvial plain elevation increasing downstream.

The alignment of valley deformation in relatively high gradient smaller streams with the very low gradient major rivers indicates that there has been minimal adjustment time since the deformation occurred, i.e., the deformation is very recent. At the same time, the various high-gradient sections show differing amounts of adjustment, indicating that this is part of an continuing process, not a single anomalous event. The nature and arrangement of the valley deformation zones indicate that the deformation is fault-based, though the nature of the faulting has yet to be determined. However, the continuation of the valley deformational pattern up to the edge of the Ouachita Mountains indicates that the faulting must have basement connections.