Rivers are complex entities that exhibit a variety of shapes and patterns. The process of channel adjustment is complex and involves a multitude of processes at varying spatial and temporal scales. Two primary dimensions of channel adjustment are planform pattern and cross-sectional shape. Streamflow and sediment control channel adjustment. The most common index of streamflow is bankfull discharge, related to the size of channel features and usually equated to the maximum flow expected to occur every one to two years. Recent studies have shown that indices controlling channel morphology vary regionally, especially in settings dominated by variable flow regimes. This is particularly important when considering spatial variability in channel adjustment along transition zones in hydrology and lithology, exemplified by the Texas Hill Country, USA. The goal of this doctoral dissertation research project is to identify the controls on mutual adjustment of channel pattern and shape within the Llano River watershed. The regional climate is characterized by a transition from semiarid to subhumid conditions, and the hydrologic regime of the region is noted for low perennial flows punctuated by extreme floods. Additionally, river sediment abruptly varies as a result of two distinct lithologies, carbonate rocks associated with the Edwards Plateau and igneous and sedimentary rocks associated with the Llano Uplift. The flashy hydrologic regime has important implications to channel adjustment in the region, particularly in examining the validity of bankfull discharge. The project asks the following questions: 1) What discharge controls hydraulic geometry for rivers characterized by flashy, flood-dominated hydrologic regimes? 2) How does the change in sediment manifest itself spatially in channel geometry for rivers of different size? 3) Is bed material or bank material more important to channel geometry? These questions will be examined through field and laboratory analyses that address the following objectives: statistically characterize flood and stream power variability; characterize downstream variability in bed material and lithology; characterize channel bank strength according to cohesion, shear strength, compaction, and calcite cementation; characterize channel pattern and shape; estimate bed-material entrainment frequency; and use statistical procedures to relate flow and sediment parameters to downstream channel adjustment.
The study contributes to a theoretical understanding of fluvial geomorphology by examining channel geometry controls for semi-arid rivers, which are underrepresented in the literature. From an applied perspective the findings are useful to an audience of engineers, hydrologists, and stream ecologists. Biologists and engineers in government agencies are concerned with the structural integrity of transportation infrastructure, reservoir sedimentation, and channel stability as related to aquatic habitat and riparian restoration efforts. This Doctoral Dissertation Research Improvement award provides critical support for a promising student to establish a strong research trajectory as an independent scholar.
