BAR SPACING IN ALLUVIAL AND BEDROCK REACHES OF THE GUADALUPE RIVER, A MIXED MORPHOLOGY OR HYBRID RIVER IN CENTRAL TEXAS

Although mixed alluvial-bedrock rivers that exhibit both alluvial tending and bedrock tending reaches are common, the research that addresses the complex morphology of these rivers is sparse. While previous research has focused on predicting the boundary conditions at a point, little research has characterized mixed channels as an objective reach type.

The Guadalupe River in central Texas is a mixed or hybrid stream channel. The channel is shallowly incised into bedrock and largely overlain by thick alluvial deposits. The distribution of channel type and gravel bars on 140 km of the Guadalupe River was mapped and the grain size distribution of bars was measured in the field. The bar spacing for the study reach is four times greater than the common range for alluvial channels. Comparisons between alluvial and bedrock reaches show that although the percent coverage by bars is similar for both types, the bar spacing is five times greater in bedrock reaches than alluvial reaches. In several locations in both channel type reaches, bars are spaced less than one channel width and are grouped together. Grain size distributions show that there is no significant difference between bars that occur in bedrock and alluvial reaches. In the study area, grain size distributions reveal neither downstream fining nor coarsening.

Mixed channels have not been considered an independent channel type, but rather, intermediate to either alluvial or bedrock dominance. Many stream channels exhibit a mixed morphology where part of the channel boundary is alluvial and part is bedrock, either in cross section or longitudinally. The Guadalupe River may be one example of mixed channel type that is objective, where bedrock processes, alluvial processes, or a combination of the two vary spatially.