CONTROL OF SOURCE AREA LITHOLOGY ON ARMOR LAYER DEVELOPMENT

In rivers where streambed sediment consists primarily of gravel, the surface sediment is often coarser than the underlying sediment. When such an armor layer is absent, the cause is typically attributed to an abundant sediment supply of relatively fine grain sizes. This research presents another reason for the absence of armor layers.

Field observations were collected from 14 rivers within south central Texas. At each site a 400 count surface sample was collected from a homogeneous unit within exposed gravel areas. Sampling subsurface sediments followed standard field and lab procedures with total field masses ranging from 260 to 690 kg to achieve 1% accuracy or better based on Church et al. (1987) criteria. Armor ratios were calculated based on the median diameters of paired surface (D50f) and subsurface sediment (D50b) derived from grain size distributions. Source area lithology upstream of each sampling site was determined using published geologic maps within a GIS. The lithology of surface clasts was also identified.

D50f ranges from 12 to 35 mm, while D50b spans a wider but slightly finer size range. While surface winnowing is evident at most sites, armor layer development varies from 1.0 to about 1.5. Where limestone is the dominant lithology, armor layers are not well developed because weathering and attrition processes produce little sand-sized material, which limits the degree of vertical size segregation that is possible. Where armor layers are more developed, sediments are derived from more varied lithologies, including those that more readily produce sand-sized grains. These results document that the development of armor layers can be constrained by the absence of fine sediments.