Equilibrium and Non-Equilibrium Adjustment of Tributary Drainages in Response to Downcutting of the Salt River, Northeastern Sonoran Desert, Arizona.

When the Salt River started flowing across a topographic low of the Mazatzal Mountains of Arizona prior to 3.2 million years ago, it lowered the base level in its tributary drainages. The basic characteristics of these watersheds suggest that they may have been close to a general state of dynamic geomorphic equilibrium. However, this inferred dynamic equilibrium does not include the topography of the Salt River canyon within about 150 m of the river that displays dramatic relief linked to differential erodibility of rock types. We interpret this non-equilibrium state to be a response to formation of a major downstream river terrace about 330 thousand years ago that then produced a wave of knickzone recession and Salt River downcutting upstream. Thus, the present-day topography of the Mazatzal Mountains draining to the Salt River consists of an out-of-equilibrium chaotic inner canyon topography below smooth landscapes in a dynamic equilibrium. [ABSTRACT FROM AUTHOR]