Pecos River north of Comstock, Texas. Properly pronounced "pay-cuss," the headwaters of the Pecos River are located north of Pecos, New Mexico, at an elevation of over 12,000 feet on the western slope of the Sangre de Cristo mountain range in Mora County. The river then flows for 926 miles through the eastern portion of New Mexico and through neighboring Texas before it empties into the Rio Grande near Del Rio. Photo by Kathy Weiser-Alexander.