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<br />~ <br /> <br />influencing the native fishes living in the Upper Colorado River Basin. However, I viewed Lake <br />Powell as the downstream boundary of the river ecosystem examined in this report (Figure 1). <br />A vital characteristic of river ecosystems is the fact that biophysical processes are inherently <br />variable. The essence of ecology is understanding the complex processes that control observed <br />variability in the distribution and abundance of biota. Quantification of the structure and function <br />of complex systems, like the Upper Colorado River Basin ecosystem, in time and space must be <br />based on long-term (> 5 years) measurements in order to detect patterns or trends that in shorter <br />time frames are overwhelmed by variability. Hence, an ecosystem approach strives to determine <br />how and why the river changes in time and space, not simply to describe current conditions. <br />Like most scientists, I view model building and logistic descriptions of dynamic events in <br />ecology as mechanistic tools for formalizing a better understanding of what is known about a <br />system; such tools should not be used in an attempt to predict the future. Predicting the <br />consequences of environmental change is the ultimate challenge of contemporary ecology. This <br />must be resolved through strong inferences based on properly scaled measurements of biophysical <br />variables that integrate the myriad of system-specific ecological processes that are spatially and <br />temporally dynamic (Magnuson 1990, Stanford and Ward 1992a). In other words, the problem of <br />instream flow provision must be resolved from strong inferences derived from long-term trends in <br />ecological processes and responses of the river ecosystem in which the endangered fishes live. <br /> <br />7 <br />