This is the second in a sporadic series of diaries about the Mississippi River. The first dealt with my personal musings about growing up in a river town. This one is a cursory overview of the geology that formed and continues to influence the river.
"THE Mississippi is well worth reading about. It is not a commonplace river, but on the contrary is in all ways remarkable. Considering the Missouri its main branch, it is the longest river in the world—four thousand three hundred miles. It seems safe to say that it is also the crookedest river in the world, since in one part of its journey it uses up one thousand three hundred miles to cover the same ground that the crow would fly over in six hundred and seventy-five. It discharges three times as much water as the St. Lawrence, twenty-five times as much as the Rhine, and three hundred and thirty-eight times as much as the Thames. No other river has so vast a drainage-basin: it draws its water supply from twenty-eight States and Territories; from Delaware, on the Atlantic seaboard, and from all the country between that and Idaho on the Pacific slope—a spread of forty-five degrees of longitude. The Mississippi receives and carries to the Gulf water from fifty-four subordinate rivers that are navigable by steamboats, and from some hundreds that are navigable by flats and keels. The area of its drainage-basin is as great as the combined areas of England, Wales, Scotland, Ireland, France, Spain, Portugal, Germany, Austria, Italy, and Turkey; and almost all this wide region is fertile; the Mississippi valley, proper, is exceptionally so. "
- Life on the Mississippi Mark Twain
During the Wisconsin Period of the Great Ice Age, 75,000 to 12,000 years ago, ice sheets up to 10,000 feet thick carved and shaped the continent. In some places that great mass of ice extended below the 40th parallel (roughly, the southern border of Nebraska or the location of Columbus, Ohio or NYC). People who live within the area covered by glaciers may or may not be aware of their influence on the surrounding terrain, but that influence was, and is, profound.
The glaciers and their subsequent melting created the landscape that allowed the mighty Mississippi River to become a major physical feature of the North American continent. Not only did the ice carve much of the basic channel of the Upper Mississippi's course, but it provided vast quantities of water which further shaped its course and the surrounding landscape. Retreating glaciers produced unimaginably large freshwater lakes, the largest of which extend over northwest Minnesota, parts of North Dakota and the Canadian provinces of Manitoba, Saskatchewan and Ontario. In view of that fact, it's no surprise that the the low spots carved by centuries of ice would fill to make Minnesota the "Land of Ten Thousand Lakes." It is one of these lakes, Itasca, from which the the Mississippi springs.
All along its length, the Mississippi collects the flow of its tributaries, the largest of which are the Missouri and the Ohio. They, in turn, are augmented by their own tributaries to the point where the Mississippi River drains all or parts of 32 states, from Rockies to the Appalachians and from two Canadian provinces to the Gulf of Mexico.
Along its course, the river carves the landscape, sometimes in strange ways. Twain remarks that even in his time, places that were in one state became separated from their state when the river chose a new path.
"IN regard to Island 74, which is situated not far from the former Napoleon, a freak of the river here has sorely perplexed the laws of men and made them a vanity and a jest. When the State of Arkansas was chartered, she controlled 'to the center of the river'—a most unstable line. The State of Mississippi claimed 'to the channel'—another shifty and unstable line. No. 74 belonged to Arkansas. By and by a cut-off threw this big island out of Arkansas, and yet not within Mississippi. 'Middle of the river' on one side of it, 'channel' on the other. That is as I understand the problem.
Whether I have got the details right or wrong, this FACT remains: that here is this big and exceedingly valuable island of four thousand acres, thrust out in the cold, and belonging to neither the one State nor the other; paying taxes to neither, owing allegiance to neither. One man owns the whole island, and of right is 'the man without a country.'
Island 92 belongs to Arkansas. The river moved it over and joined it to Mississippi. A chap established a whiskey shop there, without a Mississippi license, and enriched himself upon Mississippi custom under Arkansas protection (where no license was in those days required)."
These islands are on the Lower Mississippi, which differs considerably in character from the river in the north. Above Cairo (pronounced KAY-roe), Illinois, the river mostly carves it's channel through rocky bluffs, while downstream the terrain flattens out, allowing the river to meander with greater whimsy. Oxbows (http://en.wikipedia.org/wiki/Oxbow_lake) and cutbacks have been a fact of life on this part of the river. The water will always take the easiest path on its way to the sea. Sometimes that path changes and can form an oxbow lake when sediment fills in the ends to isolate the lake from the river. The serpentine path of the Mississippi below Cairo makes oxbows nearly inevitable. Satellite views, such as Google Earth, show the amazing wandering of the river.
This has gotten longer than I intended. It looks like floods and quakes will have to wait until next time. This is by no means a detailed vision of the geology of the river. It's a journey of discovery for me to learn more about my river. Any additions or corrections are welcomed.
References:
http://hoover.archives.gov/...
http://www.greatriver.com/...
http://www.mvn.usace.army.mil/...
http://www.gatewayno.com/...
http://en.wikipedia.org/...