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

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Purpose and Scope

This report describes a re-evaluation of the 1921 peak discharge based on three lines of evidence: (1) the n-verification studies using the 1949 flood data, (2) the n-verification study using the 2003 and 2006 flood data, and (3) the location of the 1921 peak discharge data point on the stage-discharge rating—a relation defined by current-meter measurements made at the site.

The previous n-verification studies that used data from the 1949 flood are discussed in detail by Mastin and Kresch (2005). A re-examination of those studies is discussed in this report and results of that study are the main thread of evidence for the re-evaluation of the 1921 historic flood. The n-verification study using the 2003 and 2006 flood data is discussed first in this report because the results of the field survey of the flood HWMs on November 6-7, 2006, have not been documented in any previous report and the resulting well-defined water-surface slope was substituted into the 2003 flood channel roughness verification study to re-verify the roughness coefficient. This is followed by a discussion of the stage-discharge rating curve and how this curve is important as a third piece of evidence supporting the re-evaluation of the historical discharges at the gaging station on the Skagit River near Concrete, Washington.

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