Laserfiche WebLink
The data in Figure 15a illustrate that suspended sediment concentrations in the Colorado <br />River are generally much higher on the rising limb of the hydrograph than they are on the falling <br />limb. This effect- known as hysteresis- is common to all of the gauges in the study area [Pitlick <br />et al., 1999; Pitlick and Cress, 2000]. Suspended sediment loads are likewise consistently higher <br />on the rising limb of the hydrograph than they are on the falling limb (Fig. 15b). The rising-limb <br />flows carry much higher suspended sediment loads because it is during this time (typically in <br />May) when both sediment concentration and water discharge are high. Suspended sediment <br />concentrations can reach moderately high levels at other times of the year, particularly after <br />summer thunderstorms; however, since flows are generally low at that time of year, these events <br />carry a small proportion of the total annual suspended sediment load. <br />The data set for the Cameo gauge also includes 449 measurements of the percentage of <br />sand in the suspended sediment samples. Sand includes those sediment sizes falling in the range <br />from 0.065-2.0 mm; sediment finer than 0.0625 mm is silt and clay. If the percentage of sand in <br />a sample is known, so too is the percentage of silt and clay, thus the total suspended sediment <br />load can be proportioned between the sand fraction and the silt-clay fraction. Figure 16 shows <br />the same data as in the previous figure, with the suspended sediment load split between silt-clay <br />and sand fractions. The two graphs are plotted at the same scale, thus it is evident that, in <br />general, the silt-clay fraction of the suspended sediment dominates over the sand fraction; on <br />average, 80% of the suspended sediment load of the Colorado River consists of silt and clay. It <br />is also evident in these plots that there is much more scatter in the relation between discharge and <br />silt-clay fraction than there is in the relation between discharge and sand fraction. This <br />observation indicates that amount of silt and clay carried in suspension is driven in part by the <br />supply of fines from sources outside the channel; however, the relation between silt-clay and <br />35