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not known whether a new state of equilibrium in alluvial reaches of the Green River had been achieved by the time Flaming Gorge Dam was closed in 1963. The high variability encountered in the sediment data for the Green River may be related to the effects of climatic fluctuations superimposed on the geomorphic evolution of the incised arroyos within the basin. Graf (1986) concluded that variation in the Palmer Drought Index accounted for 38 to 66 percent of the variability in the annual water and sediment yield of the Little Colorado and San Juan Rivers between 1930 and 1960. Area-averaged annual Palmer Hydrological Drought Index values (see Section 4.5) for the Upper Green River and Yampa River Basins show considerable variation for the period of record (Fig. 4.10). Five-year running means of the annual values do show trends that may help to explain some of the variability of the Green River sediment loads. From the late 19205 to about 1940 the index values indicate that this was a drier than normal period when it is reasonable to expect storage of sediment in the tributaries. The storage of sediment in the tributaries may help to explain the observed reduction in annual sediment loads in the 19405. A similar dry period can be identified between about 1950 and 1965 (Fig. 4.10). It is conceivable that the reduction in sediment load that was attributed to closure of the dam in 1963 is at least partly due to climatically induced sediment storage within the tributaries to the Green River. Regardless of the actual causes of decline in sediment loads in the rivers draining the Colorado Plateau, the fact that a period of decline was taking place before and during the time frame when Flaming Gorge Dam was emplaced casts some doubt on the equilibrium status of alluvial reaches of the Green River prior to dam emplacement. Furthermore, the possibility of continued reduction in sediment supply to the rivers as a result of ongoing geomorphic evolution of incised channels, as modified by climatic factors, suggests that both the climatology and geomorphology of the region drained by the Yampa River should be investigated in more detail if base line conditions are to be established. 6.2 Warm SpriniS Rapid and Island Park The Dinosaur River Guide (1985) states that Warm Springs rapid was formed by heavy flood waters from Warm Springs draw in 1965. The elevation drop through Warm Springs rapid is about 30 feet, and has created a large backwater area upstream. In effect, the rapid created a small dam on the river and it is reasonable to conclude that sediment storage has occurred upstream in the ponded area. Observations during the June 1989 raft reconnaissance, related to difficulties in floating/rowing the pooled area, confirm this assessment (pers. comm., Valdez, 1989). The net result of this effect is that for some time period, sediment movement, particularly bed load, was interrupted resulting in a deficit downstream. Assuming that the original bed slope through this reach was about 0.001, the backwater zone would extend upstream about 30,000 feet. Assuming an average bottom width of 100 feet, the total volume in the pooled area would be about 1,000 acre feet. Assuming that 80 percent of this volume has filled with sediment, the total deposition would be about 800 acre feet, or about 2 million tons (based on bulking of 0.7 and 165Ib/ct). This value is about the mean annual sediment loading, suggesting that the effect of Warm Springs rapid on the downstream channel was relatively short- lived; however, during this time period there was probably only limited sediment delivery downstream to the Green River. Another natural factor influencing downstream sediment movement is the Island Park reach, where the channel is slow moving and highly sinuous. This reach can be considered a sediment reservoir which may buffer upstream impacts, particularly short term changes, and minimize their significance on downstream channel conditions. For example, a short term deficit of sediment supply from the upstream channel, as may have resulted from the formation of Warm Springs rapids, could readily be made up by bank erosion and cutting through the Island Park reach. Similarly, excess sediment from upstream could be absorbed in the Island Park reach without impacting downstream conditions significantly. Therefore, the influence of Island Park is thought to be one of moderating the abrupt changes and smoothing out the variations in sediment load to downstream reaches. Permanent changes in sediment regime would 6-4 I I I I I I I I I I I I I I I I I I I