Where Cibolo Creek Sinks

Cibolo Creek crosses under US 281 here, and for much of the year the bed is dry stone. The Cibolo rises in Kendall County and flows southeast to the San Antonio River, and on the way it runs across one of the significant recharge features of the Edwards Aquifer. Where the creek meets faulted, fractured Edwards limestone, in a stretch defined for regulatory purposes as beginning at Herff Falls upstream near Boerne, it loses its flow downward into the rock rather than carrying it on to the river. In a wet year the Cibolo puts a substantial volume into the aquifer that supplies San Antonio and the springs at Comal and San Marcos. Whether it is flowing here on any given day says more about the rock than about the weather. That makes an ordinary-looking creek crossing a consequential piece of ground. Whatever is spilled on the recharge zone goes where the water goes, and quickly, because the Edwards is a karst aquifer of caves and open conduits rather than saturated sand, so very little is filtered on the way down. The country around Bulverde has been among the fastest-growing in Texas, and the argument over what may be built on the recharge zone, from quarries to subdivisions to wastewater outfalls, has run here continuously for thirty years. The creek also marks a change in the rock. Within a few miles south of it the Balcones fault system steps the whole limestone section down toward San Antonio.