Preferential Flow Pathway Controls on Groundwater Discharge in the Devils River of Texas

Groundwater in the Devils River Basin in central Texas is discharged almost entirely into the Devils River. The characteristics of groundwater discharge to the Devils River are dominated by karst features, geomorphological features, and geologic structures. Karst features are postulated to have developed in alignment with river channels over long periods of time and have resulted in preferential flow pathways or conduits. These conduits are located in close proximity to surface channels of the rivers, major tributaries, and minor channels at relatively shallow depths. The dendritic nature of the stream channel network implies that an equally complex preferential flow network exists in the basin. Recent experiments in developing conceptual and numerical models of this surface-water/groundwater system have led to several interesting observations. Permeability exists at several different scales in the study area. Discrete representation of the conduit network is required to calibrate both groundwater-level fluctuations and spring discharges to the Devils River. Conduits of low-order streams must be included in the models to replicate the base-flow response seen in the Devils River. Groundwater flow and sustainability of the Devils River watershed appears to be controlled by the morphology of the area more than the bulk hydraulic properties of the host rocks. The ability of the models to replicate Devils River discharge was only achieved when the groundwater model attained the apparently correct distribution, morphology, and alignment of conduits relative to the watershed topography. Production of groundwater in the basin results in a proportional reduction in the flow of the Devils River. The impact is most pronounced during low-flow conditions. This has large impacts on the ecology of the river system as small decreases in groundwater levels and discharge can extinguish spring clusters in the river potentially destroying refuges for fauna and flora under low-flow conditions.