The Edwards Aquifer is approximately 160 miles long measuring from Brackettville to Kyle and varies in width from 5 to 40 miles. It extends to cover the major part of five counties namely, Uvalde, Medina, Bexar, Comal and Hays. It traverses several streams in three major river basins including the Nueces, San Antonio and Guadalupe. The aquifer is a very unique carbonate aquifer located in south-central Texas. Karst characteristics of the Edwards Aquifer make it one of the most productive aquifers in the United States and yet groundwater flow within the aquifer is very complex and difficult to predict. The Edwards Aquifer is designated by the EPA as a "sole source" drinking water supply for the 1.5 million people of San Antonio and the Austin-San Antonio corridor. The aquifer is also vital to the agricultural and light industrial economy of the region. Springflows from the Comal and San Marcos Springs provide water for the tourist and recreation industry, Critical Habitat of several endangered species, appropriated water use downstream on the Gulf Coastal Plain, and the San Antonio Bay ecosystem.