During the construction of the Missouri River navigation channel, the Army erected thousands of pile dikes and revetments to narrow, deepen, and straighten the wide, shallow, meandering stream. Once the engineering works went into the river, the Missouri deposited its heavy silt-load on the downstream side of the structures. Overtime, new, elevated lands appeared in the river’s floodplain. Side channels, marshlands, and scour holes – everything that constituted the floodplain – filled with alluvium. Accumulated sediments sharply reduced the floodplain’s ability to store floodwater. Valley farmers benefitted from the newly accreted land. They expanded their operations into the floodplain, planting row crops where native vegetation once grew. The floodplain’s loss meant the farmer’s monetary gain.
By 1940, the Army had pegged the Missouri down inside a wood and rock-lined cage all the way to Sioux City, Iowa. The completion of the navigation channel coincided with the onset of a wet cycle across the Midwest and Northern Plains. High flows began to descend the Missouri with alarming regularity. But the river could not widen out. The Army’s constricted navigation channel, and the diminished storage capacity of the floodplain, prevented the Missouri from naturally reducing its flood crests. As a result, lower valley residents began to pay the price for their earlier environmental misdeeds. The Mighty Mo pounded the lower valley with costly floods in 1941, ‘42, ‘43, ‘44, ‘45, ‘47, ‘48, ‘49, ‘50, ‘51, and ‘52. Never before had the Missouri flooded so frequently and with such catastrophic results. The flood of 1943 caused $47.3 million in damages ($613.4 million in today’s dollars). The flood of 1952 cost $179 million (or $1.48 billion adjusted for inflation).
The Army recognized it had a flood problem along the Missouri – a problem it had created. If it did nothing, the navigation channel would continue to flood – wreaking havoc on towns and agricultural lands. Eventually, the river’s powerful currents and ice flows would also destroy the multi-million-dollar investment in pile dikes and revetments. Doing nothing was not an option. But curtailing the high flows through the flood-prone navigation channel could only be achieved in one of two ways – by dismantling the navigation channel and letting the river back into its floodplain or by building upstream dams and reservoirs. Unfortunately, the Army did not choose the least expensive, most environmentally sound solution. Instead, it opted to build large, expensive dams in the Dakotas. The man-made upstream reservoirs would be a substitute for the loss of natural floodplain water storage in the lower valley.
After closure of Fort Randall Dam in 1952 and Garrison in 1953 respectively, the Army rebuilt its crumbling navigation channel south of Sioux City – believing it now immune to destructive high flows. It finished the last dam at Big Bend in 1966 and the nine-foot navigation channel to Sioux City in 1981. At that later date, the Army publicly declared that it had “tamed” the Missouri.
Closure of the upstream dams led to degradation of the streambed from Yankton to the Platte River confluence. The Army referred to the clear water emanating from beneath Gavin’s Point Dam as “hungry water” because it ate away the Missouri’ bed. By 1980, the Army noted that the Missouri had degraded its bed 8.5 feet at Sioux City. Degradation, in conjunction with channelization, completely severed the links between the Missouri and its floodplain. The new, lower elevation of the Missouri drained the river’s last remaining side channels and oxbow lakes into the deepened channel. As those areas dried out and filled in with sediments, valley farmers transformed these once-plentiful wetlands to cropland. Laser leveling for efficient crop production further eliminated the valley’s undulations. By the dawn of the 21st century, the Missouri Valley south of Sioux City had lost almost all of its ability to safely store floodwaters. Its floodplain, wetlands, oxbow lakes, and side channels – its former natural reservoirs – had been plowed or paved. Consequently, when the flood of 2011 arrived in the navigation channel, it quickly overtopped its constricted, man-made banks and spread out over the flat valley – in a manner similar to spilled milk gliding across a tabletop. There were few natural depressions to slow the advancing water or diminish its height, so the Missouri moved further afield than it otherwise would have, with such dreadful results.