On May 9, 2012, the Army Corps of Engineers will cut the discharge rate from Gavin’s Point Dam to zero, yes zero. That means no water will exit through the structure’s power tunnels or spillway gates. The Army must stop the flow of water through the dam in order to inspect it for any damages. Last year, the Missouri’s powerful floodwaters pounded the structure, especially its spillway. The Army wants to know just what the Missouri did to the dam. Without water pouring through Gavin’s Point Dam, the Missouri downstream through southeastern South Dakota and western Iowa will drop to a record low level. It remains to be seen just how low the Missouri will go. Much depends on tributary inflows. If the James and Big Sioux rivers do not dump large volumes of rainwater into the Missouri, we can expect the river at Sioux City to diminish to a trickle. We do know that during the eight hours the Army pinches off the river’s flow, the Missouri will drop to one of its lowest levels ever, possibly lower than at any time since the glacial formation of the stream 30,000 years ago.
The last time the Army dropped the Missouri this low was in 1955. In late July of that year, the Army engineers slid heavy steel gates down on the river at Fort Randall Dam, permitting only 3,000 cubic feet per second to pass through that earth and concrete behemoth. With such a small volume of water coursing through its channel area, the Missouri became a mere shadow of its former self. The federal engineers dropped the Missouri to ease the final closure of Gavin’s Point Dam’s earth embankment. Gavin’s Point Dam had been rising over the Missouri since the early fifties. Closure of the embankment marked a major milestone in the dam’s construction. The engineers did not want to struggle against a strong current when they extended the dam’s embankment across the last open section of river at the dam site.
When the low water emanating from Fort Randall Dam arrived at Sioux City, Iowa, residents of that foul-smelling meat-packing town observed that all sorts of junk emerged from the river as the water line retreated behind sandbars and sand flats. Rusted automobiles, discarded home appliances, and miscellaneous debris appeared atop the exposed riverbed. The “Sioux City Journal” proclaimed that the low water level proved that “man had at last controlled the impetuous river.” ["Sioux City Journal," August 7, 1955]
In the wake of the Great Missouri River Flood of 2011, we know man did not tame the Mighty Mo, even after the construction of six of the world’s largest dams across the river’s main-stem in Montana and the Dakotas. When the river’s diminished flows silently glide past the Sioux City area late on May 9 and into May 10, the Missouri will again reveal its long-held secrets. Who knows what will be found when the river’s level reaches its nadir. Hopefully, the bed of the shrunken Missouri will not be littered with garbage from the past 57 years of human neglect, but I am not so sure.