Gila, a river of New Mexico and Arizona, the principal tributary of the Colorado river of the West. It rises in the Sierra Madre mountains in Socorro co., New Mexico, flows S. W. to near the Arizona boundary, where it bends S. and then pursues a general W. course through that territory to its junction with the Colorado, about 180 m. above its mouth. Its sources are about 5,000 ft. above the sea. The principal tributaries from the north are the Rio Nutroso, Prieto, Bonito, San Carlos, Salt river or Rio Salado, and Agua Fria creek; from the south the Rio San Domingo and San Pedro. The Santa Cruz river, after a course of nearly 100 m., is lost in the sands of the desert, and seldom discharges its waters into the Gila. For more than half its entire length, which is nearly 500 m., the Gila passes through mountains, and in some places is unapproachable, being buried between walls of perpendicular rock nearly 1,000 ft. high. It emerges from the mountains in lon. 1110 25' W., after which its course is through an open and comparatively level country to its termination. In the last 300 m. it has an average fall of 5 ft. per mile, and averages 60 ft. in width, 3 ft. in depth, and in velocity 2 m. an hour.

In the lower portion the valley is from 1 to 3 m. wide; about 150 m. from its mouth there is a considerable bend to the north, where the valley for 25 m. is from 5 to 10 m. wide. The valley is in many places covered with mezquite and cottonwood, and on its margin with the willow. Several varieties of cactus, including the pitahaya (cereus gigcin-teus), grow on the table land near the river, but never in the alluvial lands in its valley.

The ruined edifices, broken pottery, and traces of irrigating canals found along this river, show that its former population was much larger than at present. One of these structures is three stories high and in good preservation. The others are in a ruined state, and present little more than dilapidated walls, tumuli, mounds, etc, of crumbling adobe, of which the buildings were constructed. (See Casas Grandes.) About 200 m. from the Colorado, in one of the finest portions of the valley, is the reservation of the Maricopa and Pimo Indians. It is intersected in all directions by irrigating canals, and produces abundant crops. Further E., among the mountains, are many luxuriant valleys where once existed a considerable population, as is evident from the traces of cultivation and the ruins which remain.