The oldest data available on the Missouri River — from the logs of Lewis and Clark — shows that water flow on the river today is far more variable than it was 200 years ago.
The data also shows that the river today, at 500 yards across, is 220 yards narrower at St. Charles, Mo., than it was in 1804.
These changes are due to modifications of the river by the Army Corps of Engineers, said Robert E. Criss, Ph.D., professor of earth and planetary sciences in Arts & Sciences, and recent University graduate Bethany Ehlmann, who majored in earth and planetary sciences in Arts & Science and is now studying at Oxford University as a Rhodes Scholar.
Ehlmann recently presented her and Criss’ findings at the 38th annual meeting of the North-Central Section of the Geological Society of America, held in St. Louis.
The placement of wing dikes and levees, mostly after World War I, and the building of six main-flow reservoirs between 1937-1963 have created a river that Lewis and Clark would not recognize if they were here today. The structures are responsible for a deeper river that is flooding more often in recent years, the researchers said.
Lewis and Clark’s Missouri River data reveals a broader, healthier stream.
“Flood stages are getting higher over time because of restrictions that have made the river width narrower,” Criss said. “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 4- to 9-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/-5.2 inches.
In contrast, she said, at present-day Washburn, N.D. (Fort Mandan 200 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, Ehlmann said, is “due to regulation by huge, nearby main stem reservoirs.”
The conclusion from Ehlmann and Criss’ 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.”