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caused its abandonment by 1812. In 1870~ the population of the City of San Marcos was 742~ in 1900 it was 2~292~ in 1940 it was 6~006~ in 1950 it was 9 ~ 961 ~ in 1960 it was 12 ~ 7l3~ in 1970 it was 18~860~ and in 1980 it was 23~420 (Reference 1). Among the major attrac- tions are Aquarena Springs and Wonder Cave (References 2 and 3). The city is situated in the USGS Hydrologic Unit 12080005. The study area watershed is in the Water Resources Council Texas-Gulf Region~ Subregion 08~ and the Colorado River Basin. All of the streams within the City of San Marcos are part of the San Marcos River Watershed~ which is a part of the Guadalupe River Basin. Sink Creek has its headwaters in southwestern Hays County and has a drainage area of 48 square miles. As it enters the corporate limits of San Marcos, it joins with San Marcos Springs to form Spring Lake and the headwaters of the San Marcos River. The San Marcos River is formed from numerous springs from the Edwards Aquifer at the base of the Balcones Escarpment within the City of San Marcos. The San Marcos River below Interstate Route 35/U. S. Route 81 flows in a southeastern direction for approximately 70 miles to its confluence with the Guadalupe River just south of Gonzales. The total drainage area of the San Marcos River just upstream from the confluence of the Blanco River is 93 square miles. Purgatory Creek originates in the extreme southwestern portion of Hays County and northeastern Comal County. It flows to its confluence with the San Marcos River within the city and has a drainage area of 38 square miles at its mouth. Willow Springs Creek originates approximately 4 miles west of San Marcos, joins the San Marcos River just below Interstate Route 35 and has a drainage area of 3 square miles. The Blanco River originates in northeastern Kendall County, flowing east-southeast, and joins the San Marcos River approximately 1.5 miles below San Marcos. The Blanco River has a drainage area of 436 square miles. The Balcones fault zone is the dominant geological structure in the vicinity of San Marcos. It is a system of northeastward trending step faults with upthrown sides generally on the northwest. In addition to being the major geological structure in this area, the Balcones fault zone is expressed topographically as the Balcones Escarpment that separates two major land resource areas (Edwards Plateau Land Resource Area to the north and Texas Blackland Prairie Land Resource Area to the south). Elevations range from approximately 2,050 feet at the headwaters of the Blanco River to 550 feet at the confluence of the San Marcos and Blanco Rivers. 4