Since ancient times in Japan, flood control forests have been used to help protect against flooding by catching driftwood and sediment and slowing waters down before returning the water to the river through open levees. Working with the forces of nature not against them.
A good example is the Manriki forest in Yamanashi. In trials the it has been shown that the forest of large pine trees greatly reduces sedimentation and water force during large flood events. This part of the river is exceedingly steep so a normal levee wouldn't work.
When a Dutch flood control engineer was invited in the 19th century to visit the river and suggest improvements he famously said "This is not a river, it is a waterfall." (elevation graph, first red one, the greens for comparison are in order Seine, Rhine, Mekong, Amazon)
In Yamanashi, the saying goes: "Manriki to the West, Shingen Levee to the East", meaning the two flood control measures that make life near the river possible.
Here are all the nationally recognized protected forest classes in Japan. The Manriki forest falls under category 6 further down the thread.
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