Advance Copy December 8, 2003
ANALYSIS OF THE MISSOURI RIVER NATURAL HYDROGRAPH AT SIOUX CITY, IOWA
By Donald G. Jorgensen
The ‘natural’ hydrograph is said to be the hydrograph that favors restoration of native wildlife. The natural hydrograph is the hydrograph without any anthropogenic effects. A return to the natural hydrograph is typically espoused as the tool to recover endangered and threatened species. In reference to the Missouri River, the U.S. Fish and Wildlife Service’s biological opinion (2000) advocates this approach. Specifically, the biological opinion states “that a spring rise and summer drawdown must be implemented from Gavins Point Dam to restore, in part, spawning cues for fish, maintain and develop sandbar habitat for birds and fish…’. It should be noted that the proposed spring rise and summer drawdown will typically create a fall high flow, which is not present in the natural hydrograph. It is reported in the biological opinion that flow modification from Gavins Point is essential to avoid jeopardy to the endangered pallid sturgeon, the endangered least tern and the threatened piping plover. Most attention has been given to the lower Missouri River below Gavins Point Dam because the unchannelized reach from Gavins Point Dam to Ponca, Nebraska, contains sandbar islands that are a nesting sites for the least tern and piping plover. This study evaluates the natural hydrograph in reference to its relation to the welfare of the endangered and threatened species, with special emphasis on the historical reach from Sioux City, Iowa, to Yankton, South Dakota. Less than 3 years of stream-flow data (1929-1931) that have minimal anthropogenic effects are available. The U.S. Army Corps of Engineers and the U.S. Geological Survey have created a synthetic flow-hydrograph record from 1898 to 1929 and from 1931 to 1938, which were periods when the gage at Sioux City, Iowa was not operative. In addition, the Corps has created a synthetic record that adjusts for the effects of upstream dams and reservoirs on the flow at Sioux City. These records were combined to form the ‘natural’ flow hydrograph at Sioux City for the period of 1929- 1955. The flow hydrographs were used to back calculate the stage (water-level) hydrographs for the same period. Measured flow and water temperature measurements of the Geological Survey were correlated to air temperature readings. This relation was used to back calculate water temperatures for the 1929-1955 period.
The relation of temperature to the June rise was evaluated because it is commonly theorized that the fishes of the Missouri River are cued for spawning by the coupled effects of the June rise and the temperature. Special emphasis was directed toward the pallid sturgeon spawning, even though there is no reported spawning of the pallid sturgeon in the lower Missouri River, and even though there are numerous reports of spawning in tributary streams, and that there is little suitable substrate for sturgeon