Sediment concentration is influenced by the passage of sediment from far upstream, by local turbulent fluctuations, and by bedform migration. An example of long- and short-term variation in sediment concentration at the Muddy River below Clear Creek is shown in figure 17 . About 2 in. of precipitation over 2 days increased the stream discharge at the gaging station. Four cross-section measurements of sediment concentration were made during a 5.5-hour period on October 27, 1986 (K.R. Spicer, U.S. Geological Survey, written commun., 1986). The cross-section measurements showed a decrease in cross-section concentration from 19,800 to 5,900 mg/L (fig. 17A). The concentrations of the automatic samples in figure 17A recede parallel to the cross-section concentrations in a lower range of values.
Near noon on the day of sampling, 20 sediment samples were collected over a 20-minute period at a single-vertical location within the stream cross section (fig. 17B). The series of 20 samples had a mean concentration of 7,520 mg/L, with a standard deviation of 728 mg/L, and ranged from 6,280 to 9,190 mg/L. The mean concentration for the 20 samples lies along the trend of the cross-section concentrations.
Each cross-section measurement was defined by samples collected over a 10-minute period at five different centroids. The error associated with the sediment concentration curve is a composite of (1) the change in mean concentration during the 10-minute period of cross-section sampling and (2) the random fluctuations at a single location, as shown in the 20-minute series.
