'Strapped in' by wing dykes
The oldest data available on the Missouri River - from the logs of Lewis and Clark - show that water flow on the river today is far more variable than it was 200 years ago. The data also show that the river is some 220 yards narrower at St. Charles, Mo., today at 500 yards across than in 1804 when it spread out some 720 yards.
These changes are due to modifications of the river by the Army Corps of Engineers, say Robert Criss, Ph.D., professor of earth and planetary sciences in Arts & Sciences at Washington University in St. Louis, and Washington University undergraduate student Bethany Ehlmann, an earth and planetary sciences major in Arts & Sciences. Ehlmann presented her and Criss's findings April 1, 2004 at the 38th annual meeting of the North-Central Section of the Geological Society of America, held in St. Louis.
The emplacement of wing dikes and levees, mostly after World War I, and the building of six main-flow reservoirs between 1937-63 have created a river that Lewis and Clark would not recognize if they were here today. The structures on the river are responsible for a deeper river that is flooding more often in recent years, the researchers say.
"Flood stages are getting higher over time because of restrictions that have made river width narrower," said Criss. "If you make the river narrower to accommodate any given amount of flow, the river's got to get deeper."
This restriction, Criss said, can be blamed on a four- to nine- foot increase in flood stages along the lower Missouri River.
Wing dams, or wing dikes, are found approximately every 1,500 feet along the Missouri River, from outside St. Louis to Sioux City, Iowa, ostensibly for controlling the river for the barge industry.
"The ironic thing is that the Missouri River hardly has any barge traffic; most of that is on the Mississippi," Criss said. "The whole river is strapped in and the flow is much more variable now than then."
According to Ehlmann, the modern Missouri-Mississippi River confluence near St. Louis shows greater average daily stage change and greater standard deviation – 8.5 +/- 14.4 inches at St. Charles and 9.1 +/-11.1 inches at St. Louis -- than did the river mouth at Camp Dubois (near St. Louis) in the winter of 1803-04 – 5 inches +/- 5.2 inches.
In contrast, she said, at present-day Washburn, North Dakota (Fort Mandan, two hundred years ago), normal daily variability is between 1 to approximately 4 inches compared with an average stage variability of 4.1 +/- 7.1 inches when Lewis and Clark made measurements. A few days, however, show extremely large variability, greater than 20 inches. This, said Ehlmann, is "due to regulation by huge, nearby main stem reservoirs."
The conclusion from Ehlmann and Criss's study is that "flow regulation by main stem reservoirs and numerous others on tributaries does not fully offset the large increases in flood stages and greater stage variability that are caused by channel restriction and development in the lower basin," according to the researchers.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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