Mutual Adjustments between Process and Form in a Desert Mountain Fluvial System

The analysis of earth surface systems is complicated by mutual adjustments among geomorphic variables. Most statistical models used by geomorphologists implicitly assume unidirectional causation. Simultaneousequation models represent an alternative statistical approach by which mutual adjustment mechanisms can be analyzed explicitly. This technique is used to develop a process-response model of a small desert mountain fluvial system in order to examine the system's internal structure and dynamics. In general, results confirm the expected interactions among variables in the model. Discharge, size of bed material, and the type of bank material are the dominant factors directly influencing channel width, suggesting that channel form reflects an interaction among hydrologie conditions, the caliber of the sediment load, and the resistance imposed by the channel perimeter. Results also indicate that local and upstream energy conditions strongly influence the mean grain size and sorting of the streambed materials at a particular location in the drainage network, but that bed material caliber has a relatively weak effect on channel gradient. Although the model contains a self-regulatory feedback loop, the dynamics of this fluvial system appear to differ from the concept of steady-state equilibrium. When displaced from stability by an exogenous disturbance, the variable states do not return to or fluctuate around their initial values. Rather, they converge on new limits that are different from their previous stable values. This type of response suggests that the system is metasta-ble. The process-response model provides a comparative framework for future investigations of desert mountain fluvial systems.