Notes from the Field, June 27, 2011, Back-Cutting at Big Bend Dam

Fort Thompson, South Dakota.  The Missouri is flowing through the emergency spillway at Big Bend Dam.  The water shooting through the spillway gates is moving so fast and with such erosive power that it is back-cutting toward the earthen dam itself.  Left unchecked, the water could threaten the structural integrity of the dam.  Although at present, that scenario is highly unlikely.  Nonetheless, the Army is concerned about the erosion.  To address the issue, a dump truck hauled large blocks of quarried stone to the trouble spot.  The Army plans on dropping the rock atop the eroded bank sections to halt the back-cutting.  A civilian working for the Army acknowledged that the engineers did not expect to be in this predicament when they first opened the spillway gates to the Missouri’s floodwaters.  The back-cutting caught the Army by surprise.  But the military is on top of the problem, with tons of pink Sioux quartzite.Pierre, South Dakota.  The excessive outflows from Oahe Dam (located six miles upstream from Pierre and Fort Pierre) have flooded the Army’s recreation areas immediately downstream from the massive earthern structure.  The river is flowing where vacationers once picnicked or camped in the shade of tall cottonwoods.  The summer tourism season here is already over when it was supposed to have just begun.  Although South Dakota governor Dennis Daugaard recently announced that South Dakota is open for tourism, the campgrounds below Oahe Dam are going to be closed for a long time.  The water will not recede in the Pierre area until the start of the school year in late August.  Even then, the Army will have to clean-up all the muck deposited atop the campground.  The Army will also have to rebuild the destroyed facilities, including restrooms, picnic shelters, and electric outlets.  This is going to be a lost summer at recreation sites up and down the Missouri Valley, not just downstream from Oahe.  For a state as dependent on tourism as is South Dakota, this year is going to hurt.

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