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The Devils River in south-central Texas is recognized as one of the remaining pristine rivers in the state. Adding to its importance, the Devils River is a key tributary to the Rio Grande, providing essential freshwater flows to south Texas and the Rio Grande Valley. An efficient conveyance system for groundwater is shown to have formed in the karst carbonate watershed, located in a semiarid environment with modest distributed recharge, oftentimes less than 1-2 cm/yr. This conveyance system comprises preferential flow pathways that developed coincident with river channels. A strong correlation between wells with high well yield and proximity to higher-order river channels (i.e., within 2.5 km) was used as evidence of the presence of preferential flow pathways. An important observation is that groundwater flow in the Devils River watershed appears to be controlled by the morphology of the area more than the bulk hydraulic properties of the rocks. Flow measurements in the Devils River measured under relatively high-and low-flow conditions support the hypothesis that the river is gaining in downstream reaches. This characteristic leads to perennial river flow being restricted to only the lower reach of the river. Last, essentially all of the recharge to Amistad Reservoir that is derived from the Devils River watershed is contributed as surface flow from the river, and there is minimal underflow or cross-formational flow from the watershed at the point where the watershed abuts Amistad Reservoir.

The Uvalde Pool comprises the western portion of the San Antonio segment of the Edwards (Balcones Fault Zone) Aquifer. Assessment of available data on the hydro geology of Uvalde County confirms the conceptualization that the Edwards Aquifer in Uvalde County to the west of the city of Knippa acts as a partially separate ground water basin. This portion of the Edwards Aquifer is referred to as the Uvalde Pool. The Edwards Aquifer to the east of the Uvalde Pool is referred to as the San Antonio Pool. A constriction in groundwater flow between the two pools, referred to as the Knippa Gap, and marked differences in groundwater elevations on either side of the Knippa Gap are the motivation to treat the Uvalde and San Antonio Pools as sepa rate hydrogeological features. The Uvalde Pool is unique because it is the only place where the Edwards Aquifer is in hydraulic communication with the overlying and younger Buda Limestone and the Austin Chalk Aquifers. Given the karstic nature of the Edwards Aquifer in the Uvalde Pool and its relatively limited spatial extent, the Uvalde Pool is characterized as a highly transmissive aquifer, but with relatively limited storage capacity.

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