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Chapter 2 Sector Four Today The Natural Environment Geology Sector Four, along with Sectors Five, Six and Seven, is located in the Blackland Prairie to the east of the Ba1cones Escarpment. The underlying material in the area is erodible chalk and clay-shale. The base is generally covered by thick, organic rich clay soils. Most of the terrain is nearly flat. Topography Land elevations in Sector Four vary from 659.4 feet above sea level at the site of Parkview Metal Products to 554.8 feet above sea level along the San Marcos River near IH-35. The Sector Four Topography map is presented on the following page. Edwards Aquifer Zone The Edwards Aquifer is the water-bearing underground network of porous and honeycombed limestone formation of the Edwards Plateau. It is the source of water for San Marcos, as well as over 1.5 million other Texans, and is the source of the San Marcos River. The northen half of Sector Four is located in the transition zone, where faults, fractures, and other geologic features present possible avenues for recharge of surface water into the aquifer. The southern half of Sector Four is located in the artesian zone. This zone is an area of pressurized fresh water. The water is discharged through wells and natural springs. Watersheds A watershed is an area that drains into a river or creek. Sector Four lies in the San Marcos River, Willow Springs, Cottonwood Creek and the York Creek watersheds. Flood Plains There are floodways and flood plains along both the San Marcos River and Willow Springs Creek. Approximately 41 % of the land in Sector Four is located in either the floodways or flood plains. There are 135 acres of Sector Four in the 1 DO-year flood plain, described as an area which has a 1 % chance of being inundated by floodwaters in any given year. The 1 DO-year flood plain has been adopted by the Federal Emergency Management Agency as the basis for flood plain management nationwide. There are 233 acres of Sector Four in the 500- year flood plain, an area which has a .02% chance of being inundated by floodwaters in any given year. There are 105 acres of the sector located in floodways and river channels. These areas must be kept free of encroachments so that a 1 DO-year flood can be carried without substantial increases in flood height. In San Marcos, development is prohibited in the floodway, a flood plain permit is required for any development in the 100-year flood plain, and flood plain permits are not required for development in the 500-year flood plain. The Sector Four Flood Plain map is presented on the following page. 2-2