---------- Post added 06-15-19 at 07:37 PM ----------
Originally posted by goatsNdonkey Between the levee system and the lock-dam-pool system used to maintain the shipping channel, there is a great difference between our Mississippi and the wild one described in Mark Twain's book Life on the Mississippi., which sometimes made significant changes is a few areas between a river pilot's upstream passage and his next downstream passage. But then, in these high-water years, the river reminds us that it hasn't given up all of its wildness, and we forget that at our peril.
I live near the Illinois River, and geological evidence shows that during one of the inter-glacial periods, the Mississippi turned east a bit below the Quad Cities, and flowed over to what is now a middle point on the Illinois River's channel and then resumed its southerly flow down it. Had it not reverted back to the western channel eventually, I'd be living in Iowa!
Hopefully I will refrain from typing out two to three hours worth of lecture once given to Geology 101 students about the dynamics of stream flow. Here are basics concerning levees.
1) the stream economy formula (discharge = width X depth X velocity) always balances because it is a description, not a prediction or theory
2) when levees are constructed, you are trading less width for greater depth and velocity
3) consequently the stream rises higher behind a levee than it did before the levee was built, and this also increases velocity because the slope (or vertical drop) to all points downstream is increased
4) another way to think of it: levee construction creates a bottleneck causing water to backup on the upstream side (= at least temporarily) the decrease in width caused by levee construction decreases the discharge flowing past the point where the levees are built, which causes impoundment of some of the discharge approaching the levees from upstream
5) this means upstream areas will suffer worse floods as a consequence of levees constructed downstream;
6) therefore upstream towns/communities will be obliged to construct levees, which will cause worse flooding upstream of them.
etc. etc. etc........