The drainage‐basin of the Pecos River in New Mexico is a broad asymmetric trough extending from the Sangre de Cristo Mountains southward into Texas (see Fig. 1). It is bounded on the east by the westward facing escarpment of the High Plains and on the west by the crests of the Guadalupe, Sacramento and Sierra Blanca mountains, and a poorly defined divide extending northward through Gallinas Mountain and Pedernal Mountain to Glorieta Mesa. Most of the basin is formed by extensive upland erosion‐surfaces that slope gently toward the river. The valley proper, through much of its length, is relatively narrow and is bounded by steep marginal bluffs, but in some sections, notably between Acme and Lakewood (the Roswell artesian basin) and between Carlsbad and Black River, it is considerably expanded and the lowlands adjoining the river extend from 3 to 16 miles back from the ri