The Balcones Fault Zone Edwards Aquifer in south-central Texas is one of the most permeable and productive aquifers in the United States. The two largest spring complexes in Texas, Comal and San Marcos springs, flow from the Edwards Aquifer. The purpose of this study was to better define the source water for San Marcos Springs, which is located in San Marcos, Texas. These spring complexes provide habitat for seven threatened or endangered species. Defining the areas of contribution to San Marcos Springs is important for managing springflow especially during droughts. San Marcos Springs is the second largest spring complex in Texas, discharging an average of 177 cubic ft per second. Previous studies indicated that San Marcos Springs had both regional and local sources, although the relative contributions were not quantified. This study concluded that San Marcos Springs receives both local and regional flow with a greater component being derived from local sources during wet years and a greater component of regional flow during dry years. Three major fault blocks of the Balcones Fault Zone Edwards Aquifer influence groundwater flow from Bexar County eastward toward both San Marcos and Comal springs. Groundwater flows through the Artesian fault block and either discharges from Comal Springs or, depending upon stage, bypasses Comal Springs and discharges at San Marcos Springs. During dry periods, water levels in the Comal Springs and Hueco Springs fault blocks decline below Comal Springs orifice elevation, precluding local recharge, although the orifices continue to discharge water from the Artesian fault block. During wet periods, San Marcos Springs is also recharged by water infiltrating from Cibolo Creek, the Guadalupe River, Dry Comal Creek, Sink Creek, and other streams. Despite the large volume of water that these streams carry at times, they contribute relatively small amounts to San Marcos springflow.