Notes from the Field, June 23, 2011, Speedbumps in the Missouri

Adam’s Prairie Preserve, McCook Lake, South Dakota.  The Missouri, which normally flows beyond the southern border of the preserve, has moved onto the preserve itself, sinking a string of timber tracts situated along the sanctuary’s lower end.  The inundation of the forestlands has forced wildlife, such as turkeys, raccoons, and deer to seek safety on higher ground.  This evening a herd of nearly 30 deer mingled next to the engorged river.  A half mile south of the deer, the Missouri could be seen passing through tall cottonwood trees.  I wondered whether this herd had once made its home amongst those trees.  Usually deer bunch-up at food plots and in refuges during the bitter cold winter months to feed on the limited forage and to avoid the multitudes of orange-clad hunters prowling the countryside.  It is odd to see such a big herd during the heat and humidity of high summer.  In all likelihood, the flood has put the animals to flight and has forced them to congregate in a large herd.  The inundation of so much riverside habitat will put intensified animal pressure on the remaining habitat in the valley.

One argument bantered about during this flood is that the navigation channel actually is more efficient in carrying away floodwaters than the river in its pre-dam, pre-channelized condition.  This theory holds that the Army’s wingdams and revetments have straightened the river and sped-up its channel velocity.  Thus, floodwaters move much faster toward the south and east than they otherwise would without the navigation channel.  It is true that the Army straightened the river significantly south of Sioux City.  The lower river lost 70 miles in length because of the Army’s channelization work.  The Army also increased the speed of the Missouri from an average of 2 miles per hour before the navigation project to an average velocity of 6 miles per hour after the completion of the project in 1981.  Both of these factors have enabled the Army to evacuate floodwaters more quickly during normal flood years.  But this year is not normal.  The Army never anticipated a flood of this size in the lower valley.  The navigation channel was not designed to absorb the present releases from Gavin’s Point Dam.  Because the navigation channel is too narrow to safely pass the floodwater, the Missouri has overtopped and outflanked the Army’s wingdams and revetments from Ponca, Nebraska, southward.  Wingdams that had once been solidly anchored to the shoreline are now situated in the middle of the river, with water flowing up against them or over them.  Structures that once sped up the Missouri are now slowing it down.  The wingdams have become speedbumps across the Missouri’s path.  Water is abutting the wingdams, decelerating, backing up, and moving sideways onto adjacent land.  Lower valley farmers who are looking for someone or something to blame for the flood might want to consider blaming the wingdams located along their now-inundated riverfront land.

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