Variation in velocity during a 15-minute period was measured at 2 Hertz with an electromagnetic velocity meter at the North Fork Toutle River at Kid Valley on March 12, 1989 ( fig. 12 ). Flow depth was 7.1 ft, and bed material was coarse gravel. To illustrate possible variations in measured velocity during discharge measurements, a running average of velocity at mid-depth (y/d = 0.55) was calculated with a length of 40 seconds. For this example, the running average of velocity ranged more than 10 percent around the mean velocity for the 15 minute period. Conditions during storm flow sometimes require that velocity at a section be measured for intervals shorter than 40 seconds. Although velocity measurements for short intervals were sometimes necessary during extreme floods, the accuracy of discharge measurements was reduced by measuring velocity for intervals shorter than 40 seconds during storm flows. The overall accuracy in discharge measurements has been discussed by Carter and Anderson (1963) and Herschy (1979).
