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37 The statistic test was carried out using the Minitab statistical regression computing system. The results show that there was a signifi- cant difference between the rising and recessional suspended sand load, the rising and recessional Helley-Smith load, and the rising and recessional fine material concentration for 1983. Further, there were also significant differences between 1982 and 1983 recessional total suspended loads and between the 1982 and 1983 total suspended load and concentration for all the measured samples (see Table VII). Substantially more sediment was transported through the Yampa Canyon in 1983 than in 1982. If discharge was the sole factor responsible for the increase in sediment loads, then essentially no differences would be detected in the sediment rating curves. The results show that the Helley-Smith load is the same from year to year, but variation occurs between the rising and falling limbs of the hydrograph. This may be explained by the coarse sand bedload travel time from the source area at Deerlodge to the Mathers Hole sampling site. The concentration and fine material load are relatively less for a given discharge on the falling limb than on the rising limb indicating a reduction of the supply of sizes less than 0.0625 mm. The difference in total suspended concentrations between 1982 and 1983 recessional limbs was not significant, but the water discharge accounted for a substantial difference in the total suspended load. Finally, the missing rising limb measurements in 1982 may account for the difference in total suspended concentration and load when compared with 1983 data. The foregoing analysis demonstrates the variability of the concentration and sediment load on a seasonal and annual basis. Since the sediment load in the river is supply limited, large differences should be expected from year to year. Transported Sediment and Substrate Size Distributions Throughout the Yampa Canyon the bed material size is observed as a function of slope; steeper sloped reaches having boulder and cobble size substrate and the milder sloped reaches, gravels and sands. The upstream twenty miles, which are very steep with an average slope between .0024 and .0035, have angular boulder substrate. The study reach between river mile 16.5 and 20.5 has an average slope of approxi- mately .0013 and a riffle-pool sequence with substrate of predominately cobbles. The bed material from river mile 16.5 to the confluence with the Green River, excluding Warm Springs rapid, is comprised mostly of small to medium cobbles, gravels and some sand and has a slope ranging from .00079 to .0011 (Figure 2). The surface substrate of the cobble bars varies from the upstream tip to the downstream tip of the bar. The substrate D is 100 mm for the upstream portion of the RM 16.5 cobble bar and 50 mm for the down- stream portion. The combined upstream and downstream substrate samples is 70 mm (Figures 19-21). This corresponds well with the D size of 75 mm for the cobble substrate at Mathers Hole. The porosity of the cobbles is about 0.42. This is a typical value for natural grain noncohesive material (standard Ottawa sand ranges from 0.33 to 0.44). All of the cobble substrate material is locally derived from side canyon tributaries or mass wasting processes on the talus slopes and bedrock walls.