Melted snow from the past winter created an abundance of water. The Mississippi, Missouri and Ohio rivers overflowed their banks. Lakes and tributaries emptied into the rivers. Rising water created crests. The destruction of wetlands or marshes was one direct cause of the flooding.
Wetlands are phenomena of nature that have existed for eons.
The destroyed wetlands could have absorbed much of the water that overflowed. Outlining continues according to the above example and the directions given by your instructor.