Notes from the Field, June 14, 2011, Gavin’s Point Dam and the Great Flood

Yankton, South Dakota.  Today, the Army increased the volume of water released through the emergency spillway at Gavin’s Point Dam to 150,000 cfs.  Gavin’s Point Dam is the southernmost of the Army dams across the Missouri.  The Army built the low dam in the 1950s as part of the Pick-Sloan Plan for Missouri River Development.  The Army did not build the dam and its reservoir (named Lewis and Clark Lake) for flood control.  Lewis and Clark Lake is too small to check the Missouri’s floods.  Rather, the lake is designed as a re-regulating reservoir.  It is supposed to smooth out the surges of water emanating from the larger Fort Randall Dam upstream.  The Army believed re-regulation necessary to prevent damage to the valley below Yankton.  But since the start of the Great Missouri River Flood of 2011, Gavin’s Point Dam and reservoir has not smoothed out Fort Randall’s large releases.  As a matter of fact, the water out of Randall is now hitting Gavin’s Point reservoir and immediately passing on downstream through the dam’s spillway.  The water released today from Gavin’s Point represented the largest single-day flow through the dam since its closure in 1955.  All of that water is barreling down the river, eroding banklines, sinking homes, drowning crops, halting the already-miniscule barge traffic on the river, and making recreational boating impossible.  Gavin’s Point Dam is no longer fulfilling its primary mission.  Its reservoir is now incapable of re-regulating the Missouri.

A week ago, the Missouri Valley between Sioux City and Vermillion, South Dakota, remained almost entirely dry.  Small pockets of water did exist in former Missouri River oxbows or in old creek beds that once drained into the big river.  But the amount of water atop the valley did not appear out of the ordinary for June.  Today, the valley is sodden.  Water is everywhere.  Large releases out of Gavin’s Point Dam have raised the water table in the valley.  In low spots, ground water has perculated to the surface.  Elevated river levels have caused tributaries to back-up onto crop land.  Tiled fields are unable to drain into the Missouri or its feeder streams, adding to the number of pools dotting the valley floor.  Then there is the rain.  Sioux City has received almost an inch of precipitation in the past two days.  Points to the north and northwest of that city have received even more rainfall.  The rain has no where to go, so it remains in the fields.  The forecast for southeastern South Dakota, northwestern Iowa, and northeastern Nebraska, calls for a chance of rain in six of the next seven days.  If that forecast is even partially correct, the Missouri is going to go higher and the valley will be wetter a week from now.

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