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11 when water is exported completely out of a watershed and all downstream reaches are affected. Localized changes in stream flow, on the other hand, are characteristic of many small-scale hydropower projects where water is diverted through a flume or conduit to a generator at a lower elevation before being returned to the original stream channel. Similarly, temporal changes in stream flow can be either long-term or short-term, depending upon the design and operation of the facility. Flood control projects with large storage capacities can retain high flows, which typically occur during the winter and spring, for release during low-flow periods in the summer and early fall. Such modification of pre-project stream flow is on the order of months or even years. By retaining peak flows and augmenting low flows, nonhydroelectric uses (e.g., flood control, irrigation) of impounded streams often reduce the natural amplitude of water level fluctuations in tailwaters (Turner 1980). Short-term temporal changes, on the other hand, occur over the span of several minutes or hours and are characteristic of hydroelectric projects that are operated in a peaking mode. Because the demand for electricity varies over a 24-h period, water is stored during off-peak hours for generation during the period of greatest demand (e.g., during the late afternoon and evenings on weekdays; Fig. 1-1). The amplitude of the water level fluctuations below peaking projects may be equivalent to that observed in unregulated (e.g., unimpounded) streams which can be subject to enormous natural fluctuations in water level in response to rainfall and subsequent runoff from the watershed (Turner 1980). However, peaking operations often result in a dramatic increase in the frequency and rate of change of major water level fluctuations and a reduction in the duration of a given water level (stage height) in the downstream channel. Thus, the degree to which spatial and temporal flow patterns are altered is directly related to the design and operation of the