Where Slaughter Creek Sinks

US 290 crosses Slaughter Creek here, and for most of the year there is nothing under the bridge. That is not drought. It is the creek doing its job. A short way upstream Slaughter Creek runs onto the outcrop of the Edwards limestone, and from that line downstream its bed is perforated with sinkholes, solution cavities and open fractures that take the flow underground. About eighty-five percent of the water in the Barton Springs segment of the Edwards Aquifer arrives this way, through creek beds rather than by rain soaking in across the surface. Slaughter Creek alone supplies roughly seven percent of it, and Onion Creek a few miles south supplies close to a third. None of that water is lost. It is simply taking the underground route. The Barton Springs segment is small as aquifers go, about 150 square miles southwest of Austin, and it is fast. Dye poured into a creek-bed sink on this stretch has been recovered at Barton Springs within days. That speed is the reason the ground here is regulated so closely: whatever enters the creek enters the spring, essentially unfiltered, because a karst aquifer is a system of caves and conduits rather than saturated sand. The features doing the taking are all around this crossing, most of them unmarked and on private land. Whirlpool Cave, a couple of miles east, is one of the few that has been surveyed. Downstream of the recharge zone the creek fills again from other sources, so the dry reach is a middle section rather than an ending.