In many arid landscapes, riparian sites are the only places wet enough to support trees. The vertical structure of trees and shrubs is critical to many riparian habitat and aesthetic values (Brinson et al. 1981). Thus, woody vegetation is often an important objective and success measure for riparian restoration. Effective restoration planning depends on some underlying model of how the ecosystem functions– what variables determine its condition, how it became degraded, and how it will respond to alternative management actions. Vegetation within a riparian system is dependent on site conditions and the processes that determine those conditions.