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SUMMARY Sediment samples were collected at six streamflow-gaging stations in the Big Sandy River basin for determining sediment and runoff source areas. Both suspended-sediment and bedload samples were collected in order to determine transport rates at several locations in the basin from which source areas were determined. The suspended-sediment concentration was plotted against water discharge and the relation was used to predict long-term suspended- sediment transport rates. The method used to determine the transport rates was a modification of a load calculation used by DeLong (1977). The transport rates, supplemented by previously collected data, were used to delineate suspended-sediment source areas. The bedload samples were collected with a Helley-Smith sampler. These samples were compared to the Einstein bedload function to determine the reliability of the Helley-Smith sampler. The results showed the bedload transport rate measured by the Helley-Smith sampler was greater than that determined by the Einstein bedload function for the East Fork and Big Sandy River data, which have a smaller range of sediment sizes. For rivers or streams where there was a large range of sediment sizes with very little sediment in the median range the inverse was true. By combining the suspended-sediment and bedload data, total trans- port rates were determined. These transport rates were then compared to the Colby method of determining total load. The Colby method gave greater transport rates at high flows and smaller transport rates at low flows than the measured total transport rates. The difference between the Colby method and the measured total transport ranged from -29.2 percent at high flows to +42.6 percent at low flows. The Big Sandy River basin was divided into two areas for determina- tion of sources of sediment and runoff because of the control by the Big Sandy Reservoir. The area upstream from the reservoir receives most of its sediment from the bed and the banks as the river flows from its headwaters to the reservoir. Run.off increased by 1 percent, but the sediment transport increased by 98 percent between stations 09212500, Big Sandy River at Leckie Ranch, near Big Sandy, and 09213500, Big Sandy River near Farson. Erodible bedrock and the sparse vegetation cover associated with a semiarid climate were factors contributing to the sediment increase. Although the Big Sandy River downstream from the reservoir is widened by irrigation return flows, the river has very little sediment load upstream from the confluence with Little Sandy Creek. Downstream from Little Sandy Creek, there is a large increase in both runoff and sediment load. The sediment comes mainly from the Pacific Creek drainage basin. The major causes of the large sediment load in Pacific Creek are the erodible basin material and the semiarid climate. 49