The Pecos River originates in north-central New Mexico and flows into Texas, emptying into the Rio Grande. Its headwaters are on the eastern slope of the Sangre de Cristo mountain range in Mora County north of Pecos, New Mexico, at an elevation of over 12,000 feet. The river flows for 926 miles before reaching the Rio Grande near Del Rio. Its drainage …The Pecos River originates in north-central New Mexico and flows into Texas, emptying into the Rio Grande. Its headwaters are on the eastern slope of the Sangre de Cristo mountain range in Mora County north of Pecos, New Mexico, at an elevation of over 12,000 feet. The river flows for 926 miles before reaching the Rio Grande near Del Rio. Its drainage basin encompasses about 44,300 square miles.