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. , . . . '. , ~ 34 Colorado River. The Colorado River endangered fishes adapted to a riverbed with areas of silt, sand, gravel, and cobble. Gravel and cobble bed parts of the channel, especially riffles, are essential for spawning. Sand and silt bed parts of the channel, especially backwaters, are utilized extensively by larval and juvenile fishes. Andrews (1986) indicates that either aggregations that results in an all sand bed channel. or degradation that results in an all gravel bed channel would eliminate essential habitat for the endangered fish. Lamarra, et al. (1985) concluded that well-washed course substrate (cobble) with interstitial voids that contain little or no organic matter may be a critical factor in determining the preferred reproductive sites of Colorado squawfish. Consequently, the Service evaluated the effect of the Ruedi III Green Mountain water sales on: (1) the channel stability of the IS-mile reach and (2) the flows required to maintain substrate conditions suitable for Colorado squawfish spawning in the IS-mile reach. Channel Stability The IS-mile reach is currently in equilibrium with regard to sediment transport. Simons (1983) indicates that flow regulation from previous projects, geologic controls, and the gravel, cobble, and boulder bed nature of the stream have resulted in a channel that is stable over an engineering type timeframe (on the order of 100 years). Lyons (1986) confirms that conclusion through inspection of aerial photographs for 1954, 1980, and 1986. The river channel is oversized for the present flow regime, and the armoring which has occurred significantly reduces the opportunity for degradation during high flow events. Bank erosion on the north bank has been balanced with island formation to maintain overall channel width and depth dimensions. Based on this information, changes in flows as a result of Ruedi Round II/Green Mountain water sales will not affect the stability of the channel in the IS-mile reach. Substrate Flushing Flows Simons (1983) indicates that the sediment transport capacity of the stream with the present flow regime greatly exceeds the available sediment supply. Even so, there are flows at which sediment will settle and fill the voids in the cobble substrate. The Service believes fine sediments may be deposited on cobble areas within the IS-mile reach that require periodic flushing to provide suitable spawning conditions. The Service has utilized methods recommended by Milhous (1986) to determine the flows that may be required to assure adequate flushing of fine sediments from cobbles in this reach. The methods and analyses are presented in Appendix B. Within the constraints of the data available only a broad range of flushing flows (between 10,000 and 20,000 cfs) could be determined. Once these discharges have been attained, only a few hours are required to flush