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I I I I I I I I I I I I I I I I I I I Evaluation of Existin~ Conditions Figure 5.4 can be used to evaluate sediment continuity in the study reach between Deerlodge Park and Mathers Hole under existing conditions. In this figure, the raw data for the total load in tons/day is plotted against the water discharge for Deerlodge and Mathers Hole. Based on this fIgure, three scenarios pertaining to whether the channel is aggradational, degradational or in equilibrium can be developed. If the study reach were aggradational, the supply of sediment at Deerlodge Park would be greater than the amount of sediment leaving at Mathers Hole. In such a situation, the channel in the reach would be overloaded with sediments and sediment would be transported out of the study reach at the transport capacity. For this scenario, the sediment input at Deerlodge Park would necessarily be greater than at Mathers Hole, and there should be a distinguishable relationship between sediment discharge and water discharge at Mathers Hole. The second scenario is the case of a degradational reach. For this case, the sediment leaving the reach would necessarily be greater than the supply of sediment entering the reach. Consequently, for this case the raw sediment data and the rating curve at Mathers Hole should plot noticeably above the rating curve for Deerlodge. Again, the sediment rating curve at Mathers Hole should exhibit a distinguishable relationship between sediment discharge and water discharge. A possible third alternative, implies equilibrium conditions. For this scenario, no net aggradation or degradation would be occurring in the reach, however, by definition, sediments are transported through the reach at the transport capacity in accordance with the defInition of an equilibrium channel. In this case, both the input and output sediment rating curves should exhibit a distinguishable relationship between sediment discharge and water discharge. From the plot of the raw data at Deerlodge Park and Mathers Hole (Figure 5.4), it is obvious that the data are widely scattered. This scatter is greater than is apparent given that the log scale compresses the data. With the exception of flows less than 2,000 cfs at Deerlodge Park, neither the sediment data at Deerlodge Park nor at Mathers Hole indicate that there is a distinguishable relationship between sediment discharge and water discharge. Generally the data varies over nearly two orders of magnitude for a given discharge. Furthermore, comparison of the raw data at the two sites also reveals that there is no significant difference between the data at Mathers Hole or at Deerlodge Park. From this, it is concluded that there is only a weak relationship between water and sediment discharge at both Deerlodge Park and Mathers Hole. Such a weak relationship is indicative of supply limited streams where sediment transport depends to a greater degree on the watershed supply and other hydrologic factors than on re-entrainment of sediments derived from the bed and bank of the channel. As Colby (1956, p. 84) notes, mainstem locations near tributaries where sediment and water delivery by the mainstem and tributary are significantly different generally constitute an "...unpromising sediment station from which to compute sediment discharges from sediment rating curves." This situation defmitelyexists at Deerlodge Park at the Yampa-Little Snake confluence (see Andrews (1978) and Figure 2.1). Also, ephemeral streams that drain areas of widely dissimilar sediment-producing characteristics "may be entirely unsuitable for computations from sediment rating curves unless adequate adjustments can be applied (Colby, 1956, p. 157). While the Little Snake River can be described as intermittent rather than ephemeral, it drains areas of widely varying geology and sediment producing characteristics (see Section 6.3). Considering these factors the data scatter evident in Figure 5.4 is not surprising. 5-11