The Saint David 7½' Quadrangle is located approximately 55 miles (88 km) west of downtown Tucson. The map area encompasses the town of Saint David, a portion of the San Pedro River corridor, and the western piedmonts of the Little Dragoon and Dragoon Mountains. The San Pedro River flows north from Mexico to the Gila River. It occupies a broad valley inset below adjacent Pleistocene piedmont alluvial fans. The river is incised several meters below adjacent terraces and recently abandoned floodplains. Entrenchment occurred between 1890 and 1908 as a result of a series of large floods in conjunction with changing land uses and population growth (Hereford, 1993). Once entrenched the channel widened to its current width and eventually stabilized in the mid-1950s (Hereford, 1993).