how can we prevent the best river floods?

Prominent levee systems have been built as a water conservation and flood control project. 
The Mississippi River and Sacramento River in the United States, and the Po, Rhine, Meuse River, Rhône, Loire, Vistula, the delta formed by the Rhine, Maas/Meuse and Scheldt in the Netherlands and the Danube in Europe. The Mississippi levee system represents one of the largest such systems found anywhere in the world