HYDROGEOLOGIC RESPONCE TO MAJOR FLOODS ON THE BLANCO RIVER AND EDWARDS - TRINITY AQUIFERS, CENTRAL TEXAS, U.S.A

The Blanco River in central Texas flows across the karstic Trinity and Edwards Aquifers, and is a source of recharge to, and spring flow from, both aquifers. The hydrology of the river is dynamic, characterized by alternating reaches that lose flow as recharge and gain flow from perennial and ephemeral springs, some of which provide habitat to threatened and endangered species. Ultimately, much of the flow is lost as recharge to the Edwards Aquifer, contributing to flow at both San Marcos Springs and Barton Springs.

Rivers in central Texas are prone to extreme flooding, including the Blanco River. Two recent flood events on the river had varied effects on the interaction between the surface water and groundwater, as a result of different antecedent conditions and location of the most intense precipitation in the watershed. The Halloween flood of 2013 (peak stage = 35.7 ft. / 101,000 cfs at USGS gage in Kyle) occurred during the latter stages of a prolonged regional drought. More than 15 inches of rain fell in a few hours in the lower reaches of the watershed, proximal to the Edwards Aquifer recharge zone. The Memorial Day weekend flood of 2015 (peak stage = 44.9 ft. / 175,000 cfs at USGS gage in Wimberley) occurred after an abnormally wet winter and spring, including record monthly rainfall for May in Texas. The most intense precipitation in the 2015 flood occurred in the upper watershed reaches, upstream of the area that provides significant recharge to the Middle Trinity Aquifer. While both floods were extreme, the Memorial Day flood of 2015 was particularly devastating, shattering previous flood stage and flow records. In one location, the river rose over 48 ft in less than three hours.

Following both flood events of 2013 and 2015, detailed gain/loss flow synoptic surveys were conducted along ~100 km of the river after the flow conditions had stabilized. Results of these surveys indicate varied responses to the flood in different reaches. When compared to the 2013 event, the 2015 flow conditions receded slower, a significant Trinity spring began to flow (> 40 cfs), known swallets in the Edwards and Trinity rejected recharge, and groundwater levels rose higher. Antecedent conditions and the locations of the rainfall strongly influence the response of surface and groundwater in karst environments, even for extreme flood events.