INVESTIGATION OF CHANNEL DEGRADATION 1991 UPDATE MISSOURI RIVER GAVINS POINT DAM TO PLATTE RIVER CONFLUENCE

1. PURPOSE. This report is the fifth in a series of updates to Volume IV, supporting Technical Report, Missouri River Degradation. Volume IV is a part of the August 1981 Review Report of the water and Related Land Resources Management study for Metropolitan Sioux city and Missouri River Iowa, Nebraska, and South Dakota. This report, along with future updates, will provide the information necessary for proper management of the Missouri River in reference to degradation. 2. SCOPE. This report incorporates water surface profile and hydrographic survey data from the 1989 and 1990 water years for the purpose of updating the existing studies. Analyses of the historical trends, groundwater and oxbow relationships, and degradation trends are continued from the main report and the 1983, 1985, 1987, and 1989 updates. In addition, a 1987 analysis of the impacts on tributaries of the Missouri River from degradation is summarized. The study reach extends from Gavins Point Darn to the Platte River confluence. 3. AUTHORITY. The Omaha District operates and maintains a navigation project from Sioux City, Iowa to Rulo, Nebraska under the authority of the River and Harbor Act of 1945. 4. RUNOFF BACKGROUND. The runoff upstream from sioux City, adjusted to the 1949 level of depletion for the calendar year 1989, was 17.7 million acre-feet (MAF) , 7.3 MAF below normal and the 15th lowest runoff since 1898. Runoff in 1989 was only 71 percent of normal due primarily to low mountain snowpack and extremely dry subsoil moisture conditions throughout most of the upper basin. Although the 1989 runoff above Sioux City was only 71 percent of normal, the distribution in the six river reaches above Sioux City varied greatly from 133 percent of normal above Gavins Point to 66 and 85 percent of normal in the reaches above Garrison and Fort Peck, respectively. The Oahe reach experienced only 51 percent of normal runoff which was much above the 17 percent experienced in 1988. Drought conditions improvedsomewhat in the spring of 1990. The majority of the basin received near normal precipitation during March through May 1990. The total runoff from August 1989 to July 1990 upstream from Sioux City was 18.2 MAF, 3.5 MAF below normal. Overall, climatic conditions in many parts of the Missouri River Basin were comparable to the severe drought years of 1934 and 1936