Pecos, river, 926 mi (1,480 km) long, rising in N N.Mex. near the Truchas peaks and flowing SE across E N.Mex. and W Tex. to the Rio Grande; drains c.38,300 sq mi (99,200 sq km). In New Mexico, dams at Alamogordo, Avalon, and McMillan serve the Carlsbad reclamation project (est. 1906), which irrigates c.25,000 acres (10,120 hectares); in W Texas, Red Bluff Dam forms a reservoir on the Pecos. Long-standing interstate disputes about water use were settled in 1949, when a federal bill provided for a compact between New Mexico and Texas. In the heyday of ranching in W Texas, "west of the Pecos" was the term for the distinct and rugged region of the western tip of the state.

Pecos (pā´kəs), city (1990 pop. 12,069), seat of Reeves co., W Tex., on the Pecos River; inc. 1903. It is a railroad and highway junction and the market for an extensive ranch and irrigated farm area; water is supplied by the Red Bluff Dam on the Pecos. It is also a sand and gravel and gas and oil center. There are cattle feed lots. The annual rodeo, held there since 1883, was the world's first.