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Low Water on the Mississippi and Thebes Gap

Thebes Gap:

Alexander County, Illinois

“Here is a vast ledge of rocks, which stretch across the river in a direct line. The best channel in the middle of the river, in which place in low water, there is not more than six feet over the rocks.”–Zadoc Cramer, 1814

Rock Formation in the Middle Mississippi at Thebes Gap, 2006

Rock Formation in the Middle Mississippi at Thebes Gap, 2006

The Mississippi is running very low. After the Flood of 2011, the river drained away very quickly and the rain stopped. By the Summer of 2012, we in the Midwest were well into the Drought of 2012 and the river was showing the effects. Now in January 2013, the Upper Mississippi is frozen and the system of locks and dams is retaining water north of Alton, Illinois. On the Missouri a similar system of dams is retaining water in South Dakota and too little water is flowing into the Mississippi to maintain water levels for the 9-foot navigation channel on the Middle River.

Look at any aerial photograph of the confluence of the Missouri and Mississippi. The muddy Missouri spews a flume of silty water into the relatively clear Mississippi. They flow side by side downstream--the Missouri on the west, the Mississippi on the east--for several miles. During the very cold winter of 2000-2001 it was possible to see this phenomenon from the Illinois bank near the spot where Lewis and Clark started their journey up the Missouri: Lock and Dam 26 at Alton trapped ice coming down the Mississippi. South of the dam the river flowed free of ice, but ice did flow out of the mouth of the Missouri. At the confluence the two rivers, the icy Missouri and the ice-free Mississippi flowed side by side in the Mississippi channel.

Look at any aerial photograph of the confluence of the Missouri and Mississippi. The muddy Missouri spews a flume of silty water into the relatively clear Mississippi. They flow side by side downstream–the Missouri on the west, the Mississippi on the east–for several miles. During the very cold winter of 2000-2001 it was possible to see this phenomenon from the Illinois bank near the spot where Lewis and Clark started their journey up the Missouri: Lock and Dam 26 at Alton trapped ice coming down the Mississippi. South of the dam the river flowed free of ice, but ice did flow out of the mouth of the Missouri. At the confluence the two rivers, the icy Missouri and the ice-free Mississippi flowed side by side in the Mississippi channel. In the Winter of 2013 too little water is spewing out of the Missouri to feed the Middle Mississippi.

 Thebes Gap is the geological break point between the Upper Mississippi and the Lower Mississippi. The Upper Mississippi flows through a rocky gorge from Minneapolis to Thebes Gap. South of there the Lower Mississippi meanders across an alluvial plain.

At the beginning of the glacial age, the Lower Mississippi flowed along the western valley wall through an alluvial floodplain in the Western Lowlands along the Black, White, and St. Francis Rivers.

From The Mississippi: “Geologists have speculated that the river abandoned its alluvial valley and diverted through Thebes Gap, a narrow bedrock canyon in the Benton Hills, through the series of glacial floods at the end of the Wisconsinan age. At the beginning of the twenty-first century, seismologists questioned why the Mississippi would abandon its comfortable alluvial valley to ream a new course through bedrock. They noted that fault lines in the Benton Hills were active 10,000 years ago, and speculated that an earthquake along fault lines in the Benton Hills opened the canyon that is Thebes Gap. Glacial River Warren, which broke out of a glacial lake that covered northern Minnesota and North Dakota and reached north into Canada,  thundered through it, and deposited a classical alluvial fan at the mouth of the canyon.”

 “Thebes at the head of the Grand Chain and Commerce at the foot of it were towns easily rememberable as they had not undergone conspicuous alteration. Nor the Chain, either–in the nature of things; it is a chain of sunken rocks admirably arranged to capture and kill steamboats on bad nights.–Mark Twain, Life on the Mississippi

Thebes Gap, where the Mississippi takes a wide turn into the narrow reach of Thebes Gap.

Thebes Gap, where the Mississippi takes a wide turn into the narrow reach of Thebes Gap.

Mark Twain knew Thebes Gap, and while it is no longer killing steamboats, this winter modern tows can’t get through this narrow gorge between the Upper Mississippi and the Lower Mississippi.

The U.S. Army Corps of Engineers have contracted with Newt Marine and Kokosing, a marine engineering firm out of Iowa and Michigan,  to remove the rocks from Thebes Gap. When they are done there on January 11, they will move on to Grand Tower.

A tow steams passed Tower Rock at the beginning of December.

A tow steams passed Tower Rock at the beginning of December.

The river level today at Chester, Illinois is -0.6 feet, which means it is possible to walk out to Tower Rock and see just how low the Mississippi is.


[i]             Cramer, 173; Harrison, Richard W., “Report on Investigations of the Benton Hills, Scott County, Missouri, in Midwest Friends of the Pliestocene, 42nd Annual Meeting, 19-21 May 1995, 7.3; Harrison, Richard W., “Mid-Continent Urban Corridor Mapping Project,” USGS Project No.: 7160-11, U.S. Geological Survey, http://erp-web.er.usgs.gov/reports/annsum/vol40/cu/harrison.htm; Elfrink, Neil, “Gujarat Analog Response,” Archives of Central U.S. Earthquake Hazard MailList, March 21, 2001, http://clifty.com/hazard/archives/1010302-021954.html; Guccione, Margaret, “Re: ‘Gujarat Analog,’” Archives of Central U.S. Earthquake Hazard MailList, Feb 16, 2001, http://clifty.com/hazard/archives/1010216-111758.html.

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