Water discharge was measured using standard methods described in U.S. Geological Survey publications (Buchanan and Somers, 1969; Rantz and others, 1982). Depending on the stream width and the equipment used, the time to complete a standard discharge measurement could range from less than 30 minutes to more than 1 hour. During storm flows, completion time was often reduced by (1) reducing the the velocity-measurement interval to less than 20 seconds and (2) reducing the number of vertical sections to less than 20.
Reels of suspension cable were mounted on cable cars or wheeled cranes for making discharge and sediment-concentration measurements at flows too high for wading. Depth on a cable reel is measured to the nearest 0.1 ft, although unstable streambeds near Mount St. Helens were often difficult to detect to that precision with sounding weights.
From a continuous record of gage height, a continuous record of water discharge can be calculated using the prevailing stage-discharge relation. Relations between stage and discharge were derived by frequent measurements and were adjusted regularly to accommodate cross-section changes in the unstable channels. Computations of water-discharge records are discussed extensively by Rantz and others (1982).
The term "water discharge" generally is used to express discharge rates of low-concentration flows. At high sediment concentrations, however, fully 5 to 10 percent of the streamflow may consist of sediment. The term "stream discharge" is used herein to mean that the discharge rate includes the entire water-sediment mixture.