For most of the 20th century, San Antonians saw southern Bexar County as the place where sewage paused, but people didn't, on the way to the coast. The band south of the city was, and for the most part still is, a rugged, sparsely populated area of ranches and oil fields, with a scattering of small communities along the Medina and San Antonio rivers. The area's image, however, was shaped by Mitchell Lake. It had been prized as a duck habitat for centuries, but in 1901 the city bought it for use as a sewage dump, then after 1930 as...