"They modeled all 1.25 million square miles of the Mississippi River and surrounding areas on more than 200 acres with the sole purpose of understanding how it would flood and which techniques worked best and how they affected things up/down river. Years earlier, they had anassed several failed large scale prevention/levee projects."

The way you wrote this---implies to me that they either misunderstood the model or the the model gave them flawed data. Or perhaps, that they got good data and ignored it (lol). I'm curious: which was it?

"Years earlier, they had amassed...", before building the model.

The model came later as a result of the failed projects, realizing that a symptomatic approach was flawed. The model was to take a more holistic methodology to addressing the flooding along the Mississippi.

The timeline is as follows:
the Great Mississippi Flood of 1927:
Flood Control act of 1928-
Army Corps of Engineers gets to work
Sec. of Commerce H.Hoover directs Flood Relief
towns & cities which had flooded get levees
The Great Flood of 1937:
towns downstream of newly protected communities get flooded.
ACE begins with simple models in dirt
1943 gets funding to build largest scale model for study:
1"=1000' horizontal, 1"=100' vertical
German POWs used for initial labor.

sorry that wasn't clear enough before. There was no model before.

Aha---so the model was because their previous efforts made things worse---and they modeled it so they could be smarter. Thank you!

Stopping a Disastrous Plan with Science: the Bay Model has been added as a related post - related requested by Mekanikal on that post.