Sampling and measuring procedures were chosen to provide data for calculation of "total sediment discharge." The term "total" distinguishes the calculated discharge from a related quantity, "suspended-sediment discharge." Sediment is transported in streams by nearly continuous suspension of fine fractions, by intermittent suspension of coarser fractions, and by tractive movement of sediment that is too coarse for suspension by existing flow conditions.
The location of sediment above the streambed determines how the transport rate of a particular population of sediment grains is measured. Continuously suspended silt, clay, and fine sands, and intermittently suspended grains of coarse sand, are collected by depth-integrating suspended-sediment samplers (Guy and Norman, 1970). Suspended-sediment concentration at the stream cross section is defined by collecting depth-integrated samples at multiple points across the stream. The suspended-sediment concentration does not, however, represent the transport rate of grains in the "unsampled zone," at and near the bed where the nozzle of the suspended-sediment sampler does not reach.
The transport rate of sediment moving in this unsampled zone (extending about 3 in. above the bed) can be estimated mathematically from measurements of flow conditions and grain-size distributions of both suspended sediment and bed material (for example, Einstein, 1950; Colby and Hembree, 1955; Stevens and Yang, 1989). The sum of suspended-sediment discharge in the sampled zone and sediment discharge in the unsampled zone then provides a measure of total sediment discharge. The unsampled sediment discharge is usually a small percentage of the total sediment discharge, but the flow capacity of alluvial channels is influenced by sediment movement in the unsampled zone. Data required for most calculations of total sediment discharge include:
Bedload-discharge rates can be estimated from bedload samples (Hubbell, 1964; Helley and Smith, 1971), and the rates are roughly equivalent to sediment discharge in the unsampled zone. Bedload was sampled using equipment described later under "Instrumentation." Comparisons between computed bedload discharges and sampled bedload discharges were given by Hammond (1989).