In the first place, the river simply hasn't had enough time to change its ways here. Second, the geological conditions the river has to cope with have resisted its best efforts to make a shortcut. The following map shows that the Missouri flows through shale bedrock at the edge of the Pleistocene glacial zone.
Third, the neck of the oxbow is high enough--180 to 200 feet--that it has never been topped by springtime floodwater. Moreover, as shown in the Missouri River Commission map on the next page, there is a sheer cliff of indurated shale in the crook of the upstream bend, which impedes erosion. Finally, when Big Bend Dam was completed 15 miles downstream in 1966, the river above it became a deep 80-mile-long lake, and the old annual assaults of spring runoff on the banks of the neck were considerably diminished.1 (Jim Wark took the photograph on the preceding page in May of 1999, when the reservoir was at full pool for the season.)
Of course, if the Big Bend were on the commercial shipping channel that begins downriver at Sioux City, Iowa, the Corps of Engineers would have been obliged fifty years ago to cut through the neck for the convenience of barge traffic. Incidentally, a suggestion was once made to tunnel through the neck and install a turbine to generate electricity, but the drop, or "head," would have been only one foot, and so the benefits would not have offset the cost.
Daniel B. Botkin, Passage of Discovery: The American Rivers Guide to the Missouri River of Lewis and Clark (New York: Penguin Putnam Inc., 1999), 104-111.
John Paul Gries, Roadside Geology of South Dakota (Missoula, Montana: Mountain Press, 1996), 81-87.
1. The reservoir, called Lake Sharpe, was named for Governor Merrill Q. Sharpe of South Dakota, who was a forceful advocate for the building of four dams on the Missouri within the boundaries of his state (see map): Gavins Point Dam at Yankton (1952-57), Fort Randall Dam at Pickstown (1946-54), Big Bend Dam at Fort Thompson (1959-66), and Oahe Dam at Pierre (1948-62). They impound a total of 442 of the 500 miles of the Missouri River in South Dakota.