rating curve allows the discharge to be obtained for a gage height, such as the daily stage measurement. However, the stage - discharge relationship changes over time as the result of many factors, especially if there is not a well-established control. However, a procedure to “adjust” the gage readings using measurements and time allows the discharge to be determined with good accuracy for different times. The stage data has been greatly affected by changes to the river over time. For example, not only has the closure of each upstream dam changed the water discharge at Sioux City, but also each closure has changed the sediment discharge, especially the closure of Gavins Point Dam in 1955. The closure of Gavins Point initiated strong streambed degradation. The channelization of the river at Sioux City up to Ponca also strongly changed the stage relationships, especially after 1964 in that reach. These changes are shown in figure 6 (Swoboda, 2003, written com.).
Figure 7. Missouri River stage trends at Sioux City, Iowa (from USACE). 1970 gage datum used, which is 20 feet lower than the pre-1970 datum.
Calculating the stage for the synthetic hydrograph is not straight forward. A rating curve developed for the stage - discharge measurements from 1928-1931 would have merit because the actual hydrograph and the synthetic hydrograph are