The San Marcos River in south-central Texas (USA) is a major tributary to the Guadalupe River that empties into the Gulf of Mexico. Its 3,520-km2 watershed drains central Texas and the lower extent of the Great Plains. The San Marcos River is well-known for its hydrological, biological, and cultural resources. The Spring Lake headwaters is the perennial origin of the river, responsible for a median flow of approximately 110 million gallons per day (170 cfs). The water from these springs is a consistent 23° C, is well above the EPA standards for clean drinking water, and has never stopped flowing in recorded history. The immediate region of the springs has provided archaeological evidence suggesting it is one of the longest continuously human-inhabited sites in North America at more than 12,000 years old. People of this Coahiultecan region claim historical residence and are survived and represented by the current Miakan-Garza band who operate the local Indigenous Cultures Institute. The aquatic ecosystem of the San Marcos River is one of the most biodiverse in the southwestern USA, and contains eight known endangered/threatened species, some of them endemic.
The San Marcos River is primarily fed by the Edwards Aquifer, an extensively regulated karst limestone system. Current and future pressures exerted upon this valuable water resource include two of the fastest growing metropolitan areas in the USA, San Antonio to the south (2.4 million people) and Austin to the north (2.1 million people). Caught in the middle of this emerging mega-region, San Marcos was the fastest growing city in the USA (by percentage) from 2012 to 2015, more than 8% growth each year. The Texas State University campus in San Marcos has also grown rapidly and now consists of 39,000 students. San Marcos and its river are a major tourist destination, receiving over a million visitors each year.
While the biophysical system of the river has received much attention, regulation, and research, the social demands placed upon the river have received much less attention. We have conducted thousands of multi-modal surveys of students, residents, and tourists visiting the San Marcos River and found that the regulating services of river habitat and clean water were the most valued ecosystem services; yet cultural services played a key role in providing direct experience with the river. Ongoing studies highlight the importance of stakeholder use in shaping and defining both individual and community values associated with the social-ecological system of the San Marcos River.