Rapid changes in cross-sectional area were measured with depth-sounding weights at several gaging stations during storm flow in 1980-81. At the South Fork Toutle River at Toutle, the cross section at the bridge was measured six times in 27 hours on November 7-8, 1980 ( fig. 13 ). Plots of gage height, discharge, and mean bed elevation versus time show that river stage was an unreliable indicator of discharge during recession of the November 7-8, 1980, storm flow ( fig. 14 ). Variation in the stage-discharge relation was identified by increasing the frequency of discharge measurements.
Gradual channel changes due to scour and fill also can occur over many days. The mid-channel bar at the gaging station, North Fork Toutle River at Kid Valley, often eroded during storm flow and reformed during flow recession. The mid-channel bar was composed of sand and coarse gravel. Cross-section plots ( fig. 15 ) show the rise in bed elevation over 4 days in 1986. The first cross section was measured on the day of peak discharge, and the other two were measured on flow recession. The stage-discharge relation was adjusted for the changes in bed elevation.
