USGS Map 1923
East of Silver Bell. "Said to be a Papago word meaning 'Big Plain.' " letter, P.M., Rillito. "A vast flood basin into which many arroyos run down from the surrounding mountains to lose themselves therein." Hornaday.
"On April 6, 1934, the name Avra was brought to our attention by the U. S. G. S. which pointed out that many maps gave two names for the same valley. Avra for the northern fraction and Altar for the southern. The Board thereupon decided the question as follows. Avra valley in Arizona for the northern section which drains north; its waters ultimately reaching the Gila river. Altar valley is the valley of the Rio Altar in Mexico draining southwest; its waters ultimately reaching the Gulf of California." Letter Secretary U. S. G. B., July 13, 1934.
This indicates there are two separate valleys, one in Mexico sloping to the southwest from close to the American line; the other draining to the north in Arizona.