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13 5 DISCUSSION The potential impacts identified for the four hydropower operational scenarios under consideration represent relatively minor shifts in species abundance along the elevation gradient. The largest impacts would occur in the lower riparian zone, with decreases of 40% (2.1 acres/mile) to 74% (3.9 acres/mile) for the seasonally adjusted moderate fluctuation and seasonally adjusted steady flow scenarios, respectively. However, lower-zone vegetation was not abundant prior to flow regulation and is therefore not considered an important component of the natural system. Relatively minor impacts are anticipated because all of the operational scenarios are within the operational capacity of the dam. Thus, all releases would be between 800 and 4,700 cfs, and for the most part, fluctuations in flow would occur within the existing channel that has been incised by past operations. The sediment transport modeling conducted for the four operational scenarios and historical operations revealed little difference among scenarios (Williams et al. 1995). Although a slightly greater amount of sediment (about 15%) was expected to be moved by scenarios featuring higher fluctuations, all estimates were within the range of uncertainty of the model used. This result indicates that the operational scenarios are not likely to differ in their potential to erode existing riparian areas. From a conservation standpoint, the pre-dam vegetation in the upper portion of the upper riparian zone is of concern. This portion of the riparian zone contains many of the large native trees, especially Fremont cottonwoods, that are considered important elements of the riparian ecosystem of western rivers. Several studies have documented dramatic reductions in recruitment of cottonwoods and other trees (e.g., box elder) following flow regulation, apparently as a result of changes in flooding regime and sediment transport processes that affect creation of suitable germination sites (Johnson et al. 1976; Fenner et al. 1985; Bradley and Smith 1986; Rood and Heinze-Milne 1989; Rood and Mahoney 1990). Although all of the seasonally adjusted flow scenarios would follow a pattern somewhat similar to the pre-dam flow regime (i.e., spring peak in flows followed by low summer flows), none would have flows of sufficient magnitude to significantly rework sediment deposits (Williams et al. 1995) and create the new germination sites that appear to be important for these species. Periodic spring releases at levels similar to pre-dam annual floods (approximately 7,000 cfs), followed by a gradual reduction in flow to lower summer flows, might be necessary to maintain this vegetation over the long term (Rood and Mahoney 1990).