A new article in the Journal of the Arizona-Nevada Academy of Science by a team of Arizona geologists including AZGS's Brian Gootee, has documented the existence of an "older, higher terrace" that supports the idea that "the lower Salt River originated by lake overflow from an ancestral Pliocene lake in the Tonto Basin."
They conclude that "The existence of this terrace and its distinct gravels are consistent with, but do not prove, a lake overflow mechanism for the initiation of through flowing drainage in the Salt River Valley." [right, Salt River Valley. Credit, Phoenix Valley Real Estate Blog]
Ref: Phillip H. Larson, Ronald I. Dorn, John Douglass, Brian F. Gootee and Ramon Arrowsmith, "Stewart Mountain Terrace: A New Salt River Terrace with Implications for Landscape Evolution of the Lower Salt River Valley," Arizona, Journal of the Arizona-Nevada Academy of Science 42(1):26-35. 2010, doi: 10.2181/036.042.0105