The southern Edwards Plateau, southwest Texas, is the recharge area for a thick limestone aquifer that supplies potable water to more than 850,000 people and irrigation water for more than 2,500 sq mi (6,500 sq km) of cropland. Because the increasing population effects a booming residential and recreational development in this region, information on the geological environment of the plateau must be made available to planners so that the aquifer can be safeguarded. A reconnaissance environmental mapping technique has been developed in a pilot study that is to precede a major regional mapping program. Constraints in developing the technique were plateau geology, land use, factors controlling recharge, and available materials, including aerial photography and maps. The basis for defining the environmental mapping units is dominantly geomorphic; lithologic differences are indicated where necessary. The units are: karstic tableland, karstic lowland, deeply dissected carbonate, moderately dissected carbonate, carbonate, shale, alluvial fan, terrace, flood plain, fan plain, and alluvial-colluvial material. Results of this mapping are directly applicable in land use planning.