One of the most beneficial things about a flood, that I can think of, is it's effect on the surrounding soil. As water, say a river, overflows it's banks, it spreads across the surrounding land. As this happens, the water carries with it silt, from the river bottom and soil from up stream. This mingles with the soil of the flood area. The result, after the water recedes, is soil that is revitalized. This soil has, from this process, received a fresh dose of minerals and nutrients.