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Total Transport Rate Total transport rate was determined by two methods. One method combines the measured bedload transport rate and the measured suspended- sediment transport rate. The other method, used mainly as a check for the measured total transport rate, was developed by Colby (1957). The Colby method uses a relation of unmeasured sediment discharge to mean velocity and to concentration of measured suspended sediment to predict the total transport rate. The two methods are compared in table 8 for those stations where the bedload-transport rate was measured. Calcula- tions of total transport rate using the Colby method were not made for the Big Sandy River at Leckie Ranch because this method cannot be used for gravel and cobble streambeds. The results in table 8 show that the Colby method gives larger transport rates at high flows and smaller transport rates at low flows than the measured total transport rate. However, more data are needed to compare the methods over a wider range of flow conditions. SOURCE AREAS OF SEDIMENT AND RUNOFF The sediment load of a stream generally is not supplied equally from all areas of the drainage basin. Some areas may contribute a relatively large part of the annual sediment load, while other areas contribute relatively minor quantities of sediment. Similarly, runoff is seldom supplied evenly from throughout a drainage basin. Sediment and runoff source areas commonly can be identified for a drainage basin provided the sediment loads and runoff are measured or estimated at several points within the basin (Andrews, 1978). Because the Big Sandy Reservoir forms a control on the Big Sandy River, the basin was divided into two parts for the purpose of determin- ing the source areas of sediment in the drainage basin. The relative contributions of sediment transport and runoff at various sites in the drainage basin in relation to the totals at station 09213500, Big Sandy River near Farson, and at station 09216050, Big Sandy River at Gasson bridge below Eden are shown in figure 28. Although there is very little runoff gain between stations 09212500 and 09213500 upstream from Big Sandy Reservoir, there is a large increase in both suspended-sediment transport rate and total transport rate (fig. 28). Pacific Creek, with a drainage area of about 500 square miles, contributes only 7 percent of the runoff but 70 percent of the suspended- sediment load for the drainage downstream from Big Sandy Reservoir (fig. 28). The total load value for Pacific Creek consists of only suspended- sediment data because no bedload data were available. To further aid in determining areas that contribute the sediment load, samples were collected during the spring of 1976, the fall of 1977, and the spring of 1978. These results are shown in figures 29 and 30. During these periods only suspended-sediment data were collected. The results indicate that very little sediment is transported during the fall and that most of the sediment during the spring is supplied by Little Sandy Creek to the Big Sandy River downstream from Big Sandy Reservoir. 41