Oxbows and Meanders

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I found this tangled map, created in 1944, over on the ever-fruitful NASA web site for the Earth Observatory. It shows historical changes along a stretch of the Mississippi River.

North of the Atchafalaya River. The 1999 satellite image shows an oxbow lake from 1785, created when a meander (a bend in the river) closes itself off to leave behind a crescent.  From the Geological Investigation of the Alluvial Valley of the Lower Mississippi River, published by the Army Corps of Engineers in 1944. Source: Earth Observatory

North of the Atchafalaya River. The 1999 satellite image shows an oxbow lake from 1785, created when a meander (a bend in the river) closes itself off to leave behind a crescent.
From the Geological Investigation of the Alluvial Valley of the Lower Mississippi River, published by the Army Corps of Engineers in 1944.
Source: NASA/Earth Observatory

I stumbled upon it while looking at a small collection of river surveys from 1865, and comparing them to modern Google maps. There was this one, a stretch just south of St. Mary, Missouri.

Sheet 6 of the 1865 U.S. Coast Survey Map of the Mississippi River from Cairo, IL to St. Marys, MO.  Source: Wikimedia

Sheet 6 of the 1865 U.S. Coast Survey Map of the Mississippi River from Cairo, IL to St. Mary, MO.
Source: Wikimedia

The modern one looks a bit different – fewer bends, fewer islands – but not so much that it would be unrecognizable. Notably, the large bend that once branched off to St. Mary, Missouri, visible at the top of each map, is now just a small tributary.

One might have expected more of a difference over the course of 150 years of population increase and civil engineering.

The same stretch of river, with St. Mary, Missouri in the upper left corner.  Source: Googlemaps

The same stretch of river, with St. Mary, Missouri in the upper left corner.
Source: Googlemaps

But, at least on the Mississippi, the differences in major river flow come when the river is left alone to shift, meander, silt up and sidle over. The more humans work on this particular river, the more it stays the same. Levees are installed to prevent overflow (although they don’t always work).

The entire Mississippi Delta once shifted every 1,000 years or so – but with industries and port installations firmly established over the course of a few human generations, that would be an economic disaster. The Old River Control Structure, undertaken in the 1950s, keeps the delta in place.

More or less. At least, for the time being.

Because in the long run and when left to their own devices, rivers are all over the map.

Section of the 1944 Mississippi River Meander Belt. For more, visit here. Source: VisualNews

Section of the 1944 Mississippi River Meander Belt. For more, visit here.
Source: VisualNews

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