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Sioux City from 1929 through 1931 is the least modified discharge record available and for this study is considered as part of the natural hydrograph.

DISCHARGE AT SIOUX CITY, 1929 - 1955

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Figure 3. Discharge at Sioux City from1928-1955. Data from USGS.

The USACE has created a synthetic hydrograph of daily flows at Sioux City (and other sites) that in essence corrects the discharge for the main stem reservoir affects starting with the closure of Fort Peck. The synthetic discharge was developed by passing inflow less current water use (depletions) and less reservoir evaporation at each dam. This is as close to representing natural conditions as possible without removing the dams. Such a condition would be impossible to attain, since inflow is determined from reservoir levels less outflow and on a short-term basis, reservoir levels fluctuate with wind and other things besides inflow (McAllister, 2003, written com.). Thus, the synthetic discharge record is a theoretical ‘natural’ standard useful for comparative purposes. The synthetic hydrograph is also termed ‘run of the river’ as it would be the hydrograph that would have resulted if all the dams were opened.

A comparison of the ‘run of the river’ discharge record with the USGS measured records for 1929, 1930, and 1931 can be made by examination of figures 4, 5, and 6.

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