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milieu. Examples of these parameters are species or guild abundance, richness and diversity. The functional, or dynamic, aspects of the system can be described by such parameters as production, respiration, and nutrient cycling (decomposition pathway). Interactions between system components can best be described in terms of processes or rates which reflect energy flow and the interaction within the biotic components as well as with the driving environmental variables. Thus, processes as well as structural factors must be considered as quantitative descriptors of the system. Physical or environmental variables are seldom constant, but may oscillate over time (Figure 4). Biological components of the system exist within the constraints of physical factors and respond with oscillations in populations, production, and other processes (Figure 5). Irregardless of these system behavioral changes, the trajectory of this behavior can be predicted, and one can identify a stability zone within which the variance in system behavior is expected to lie. This behavior can be used as a quantitative descriptor of the ecosystem. EXTERNAL PHYSICAL FACTORS There are four major sources or external inputs into the White River in the study area: (a) direct atmospheric contribution, (b) surface flow from the adjacent watershed, (c) upstream contribution, and (d) accrual (springs). The impact of these external sources of water, materials, and energy is both direct and indirect. Various substances enter the White River system through the air-water interface (Table 1). The introduction of these products from the atmosphere as well as from the watershed can provide the driving forces upon which this river system operates. For example, during a series of storms in March and April, 1979, 1.02 inches of precipitation fell. As a result, flows in the White River increased from 1,000 cfs to 1,800 cfs and total suspended solids increased from 200 mg/l to 4,500 mg/l, thus decreasing light penetration and decreasing net photosynthesis by the benthic algae. In general, one of the dominant overriding, external factors regulating the river is its arid climate and seasonal 267