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nearly identical at this stage. However, such a rating curve would not include the record high flows, which occurred in 1952. Thus, the rating for the highest discharges would not be supported in those ranges. Also, examination of figure 7 indicates there were differences between stage - discharge relationships from 1929 to 1931, and also differences from 1931 to 1937. These differences are not unexpected because changes in the stage – discharge relation for natural unmodified streams in unconsolidated material are the norm. The problem at hand is to determine the stage for the ‘run of the river’ hydrograph for the period of 1929-1955. Examination of figure 7 shows that for the period of 1945 to 1955 stage trends were relatively stable. The natural hydrograph had been modified somewhat by Fort Peck Dam, however, bank stabilization above the Sioux City gage had not occurred and Gavins Point and other dams had not been closed. The biggest change was that water storage at Fort Peck Dam had reduced the maximum discharges especially during the June rise.

The stage-discharge data for 1938-39 plus the high flow data from 1952 are shown in figure 8. The1938-1939 period was selected for several reasons not the least of which was that observations of the river stages that covered all sandbars were available. Additionally, the closure of Fort Peck Dam would not have significantly affected the stage-discharge relationship at Sioux City, especially by 1938-1939. The high-flow data from 1952 were added because these data are the only discharge measurements of flow greater than 200,000 cfs. It was not possible to satisfactorily fit a polynomial mathematical equation to the data that would reflect the points of inflection shown by the data, for example at 6 to 7 feet.

STAGE - DISCHARGE DATA FOR MISSOURI RIVER AT SIOUX CITY ( 1938-39 AND 1952 HIGH FLOW DATA)

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Figure 8. Missouri River rating curve at Sioux City1951 -1952 rating curve for the Missouri River at Sioux City, Iowa. Data from USGS.

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