If anyone needs proof that the Army’s navigation channel has reduced the conveyance capacity of the Missouri, they need look no further than the Army’s own satellite images posted at http://www.nwo.usace.army.mil/html/op-e/maps.html. The maps of the reach from Gavin’s Point Dam to Sioux City clearly show that in the 55-mile unchannelized river, the Missouri stays almost entirely within its banks at 150,000 cubic feet per second – the amount scheduled to be released from Gavin’s Point Dam in mid-June. The maps make it apparent that the unchannelized river is able to absorb almost all of the Missouri’s projected floodwaters without moving deeply into the surrounding valley lowlands.
At Ponca, Nebraska, the Army’s channelization structures begin to hem in the river. Not coincidentally, it’s here that the Missouri is first shown pushing far out into the valley lowlands. Downstream, where the Army’s pile dikes and revetments further narrow the river, the Missouri is shown shoving its waters far afield. Then, between river mile 738 and 733, the river breaks out of its narrow navigation channel and moves toward Dakota Dunes, South Dakota, and South Sioux City, Nebraska.
With its satellite images, the Army has provided proof that the navigation channel cannot contain the Missouri’s high flows. Thus, the Army’s pile dikes and revetments are a major contributing factor in the Great Missouri River Flood of 2011.