What is one way in which human beings try to control the flow of the mississippi?

As the rivers flow, they scour the channel in which they are flowing, creating "banks" to the river. Think of the process that created the Grand Canyon, but at far smaller scales. The banks hold the river under most conditions, but when somewhere upstream more water gets added to the system, the river can overflow its banks, submerging the land around it. That's a flood.