how can we prevent the best river floods?

Context: Prominent levee systems have been built along 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. During the Chinese Warring States period, the Dujiangyan irrigation system was built by the Qin as a water conservation and flood control project. The system's infrastructure is located on the Minjiang, which is the longest tributary of the Chang Jiang, in Sichuan, China.

The Mississippi levee system represents one of the largest such systems found anywhere in the world. It comprises over 5,600 km of levees extending some 1,000 km along the Mississippi, stretching from Cape Girardeau, Missouri, to the Mississippi delta. They were begun by French settlers in Louisiana in the 18th century to protect the city of New Orleans. The first Louisiana levees were about 90 cm high and covered a distance of about 80 km along the riverside. The U.S. Army Corps of Engineers, in conjunction with the Mississippi River Commission, extended the levee system beginning in 1882 to cover the riverbanks from Cairo, Illinois to the mouth of the Mississippi delta in Louisiana. By the mid-1980s, they had reached their present extent and averaged 7.3 m in height; some Mississippi levees are as high as 15 m. The Mississippi levees also include some of the longest continuous individual levees in the world. One such levee extends southwards from Pine Bluff, Arkansas, for a distance of some 610 km. The scope and scale of the Mississippi levees has often been compared to the Great Wall of China.

The United States Army Corps of Engineers recommends and supports cellular confinement technology as a best management practice. Particular attention is given to the matter of surface erosion, overtopping prevention and protection of levee crest and downstream slope. Reinforcement with geocells provides tensile force to the soil to better resist instability.

Artificial levees can lead to an elevation of the natural river bed over time; whether this happens or not and how fast, depends on different factors, one of them being the amount and type of the bed load of a river. Alluvial rivers with intense accumulations of sediment tend to this behavior. Examples of rivers where artificial levees led to an elevation of the river bed, even up to a point where the river bed is higher than the adjacent ground surface behind the levees, are found for the Yellow River in China and the Mississippi in the United States.

Answer: 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