Above Sioux City, Iowa, the Missouri's fluctuating flow is regulated by seven major dams (Gavins Point, Fort Randall, Big Bend, Oahe, Garrison, Fort Peck, and Canyon Ferry) and more than 80 other dams on tributary streams. These dams, with their reservoirs, are part of the coordinated, basin-wide Missouri River basin project (authorized by the U.S. Congress in 1944), which provides for flood control, hydroelectric power, irrigation water, and recreational facilities. The dams serve to impound for later use the spring rains and snowmelt that swell the volume of the river in March and April and also the second flood stage that frequently occurs in June as the snow melts in the remoter mountain regions. Despite this system of dams, during the extremely rainy summer of 1993 the lower Missouri reached record levels, flooding many areas, eroding farmland, and depositing huge quantities of sand that damaged many thousands of acres of fertile bottomland. Flooding was also a significant problem along the river in 2011.
Since the dams have no locks, Sioux City is the head of navigation for the 9-ft (2.7-m) channel maintained over the 760-mi (1,223-km) stretch downstream to the Mississippi. Tugboats pushing strings of barges move freight along this route. From December to March, navigation is interrupted by ice and low water levels (resulting from upstream freezing); summer water levels, which frequently fell so low as to cause river boats to go aground, are now maintained at safe levels by the release of water from Gavin Point Dam. Silt, fertilizers, and pesticides, which are contained in the runoff from agricultural lands, pollute the water above Sioux City, but wastes from industrial plants and from inadequately treated municipal sewage create a more serious level of pollution downstream. There has been a reduction in wetland areas and a loss of fish and wildlife due to the damming of the river.