| 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.
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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.
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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
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". | 
