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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5159

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Historic Peak Discharges Based on the Stage-Discharge Rating

Another indicator that the published historical discharges may be too high is the location of the peak-discharge data points that plot to the right of the current stage-discharge rating curve at the site. This rating was extended based on current high-flow current-meter measurements; the highest is measurement number 475 that was made on October 21, 2003, at a discharge of 138,000 ft³/s (fig. 7). The primary channel feature that controls the stage-discharge rating at this site is a bedrock gorge that is relatively immune to typical scour and fill activities common to many rating controls. The lack of scatter in the long history of current-meter measurements on the plot of the stage-discharge rating supports this conclusion of a stable rating that has not changed substantially over the last 80 years. The channel geometry does not substantially change at the highest stages, which indicates that a straight-line extension of the rating beyond the highest current-meter measurement is a reasonable method to estimate high flows. Water year 1925 discharge measurements are highlighted in figure 7 because they represent the stage-discharge relation near the time of the historic floods.

In order to revise the historical peak discharges based on a revised peak discharge for the 1921 peak discharge (228,000 ft³/s), a new rating was drawn. Using the water year 1925 discharge measurements, the highest current-meter measurements, and constraining the rating curve to pass directly through the recalculated 1921 peak discharge data point, a rating curve was redrawn and extended with a straight line to a gage height of 69.3 feet, the estimated gage height for the 1815 peak discharge (fig. 7).

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