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

The analysis o f earth surface systems is complicated by mutual adjustments among geomorphic variables. Most statistical models used by geomorphologists implicit ly assumeunidirectional causation. Simultaneousequation models represent an alternative statistical approach by which mutual adjustment mechanisms can be analyzed explicitly. This technique is used t o develop a processresponse model of a small desert mountain I fluvial system in order to examine the system's internal structure and dynamics. I n general, results conf i rm t h e expected interact ions among variables i n the model. Discharge, size of bed material, and the type o f bank material are the dominant factors directly influencing channel width, suggesting that channel form reflects an interaction among hydrologic conditions, the caliber of the sediment load, and the resistance imposed by the channel perimI eter. Results also indicate that local and upstream energy conditions strongly influence t h e mean grain size and sor t ing o f t h e streambed materials at a particular location i n the drainage network, but that bed material caliber has a relatively weak effect o n channel gradient. Although the model contains a selfregulatory feedback loop, the dynamics of this fluvial system appear t o differ f rom the concept o f steady-state equilibrium. When displaced f rom stability by an exogenous disturbance, the variable states d o not return t o or fluctuate around their init ial values. Rather, they converge o n new l imits that are different f rom their previous stable values. This type of response suggests that the system is metastable. The process-response model provides a comparative framework for future investigations o f desert mountain fluvial systems. I