Old River

A former section of the Mississippi River that would flow backwards, if allowed.

Within living memory, the Red River emptied directly into a big westward pointing meander of the Mississippi River.  Slightly downstream on the same meander, the Atchafalaya River drained a small amount of water off the Mississippi.  However, a large natural logjam blocked it.

In 1831, Captain Henry M. Shreve built a canal across the neck of the meander, hoping to shorten the steamboat trip to New Orleans.
 

\\R              |   |
 \\e             |   |
  \\d ___________/   |
   \\/               /
    \    _________  /
    |   /         ||
    | M |         ||Canal
    | i |         ||
    \  s \________||
    //\ s i s s i  \
   //  \________ p  |
  //            \ p |
 //Atchafalaya  | i |
||              |   |
 

The Mississippi River changed its course to flow through the canal, the northern half of the loop dried up, and the southern half, now called the "Old River" carried the Red River into the Mississippi. Well, most of it, anyway.   When the Red river flooded, most of the flood went down the Atchafalaya.  When the Mississippi flooded, the Old River reversed its flow and most of the flood went down the Atchafalaya River, too.

\\R              |   |
 \\e             |   |
  \\d            | M |
   \\            | i |
    \\           | s |
    | |          / s /
    | |         | i |
    | |         | s |
    \  \        | s |
    //\ \_______| i |
   //  \O L D     p |
  //            \ p |
 //Atchafalaya  | i |
||              |   |

In 1839, the State of Louisiana had the bright idea to dig out the logjam, in order to develop southwest Louisiana by providing a shorter navigable path to the sea! The river liked this idea, and the Atchafalaya grew and grew, taking more and more water from the Mississippi.  The Red River was now tributary of the Acthafalaya, not the mississippi. If this had been allowed to continue, New Orleans would have eventually been left high and dry. Well, as high and dry as a city in a swamp below sea level can be. At any rate, the river wouldn't be navigable, and with no ships, no city¹.

So, in 1951, the US Army Corps of Engineers began the construction of a dam on the Old River, plus a spillway to carry Mississippi floods over into the Red River and out the Atchafalaya. The Corps attempts to keep 70% of the Mississippi's flow going down the lower Mississippi.

These structures hold back the river, at least until Old Man River decides he wants to flow a diferent way.

With help from
http://www.mvn.usace.army.mil/pao/oldriver/problem.htm

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¹The Mississippi River Delta is constantly replenished by the sediment from half a continent.  Without this sediment, the Gulf of Mexico would soon erode the natural levees and barrier islands separating it from the city, giving a whole new meaning to "Mardi Gras floats".