Floods usually happen after massive storms. The most publicized floods in recent times — Pakistan today, Katrina, Aceh — certainly follow that pattern. The human toll is terrible. But extreme weather of the sort we’d better be primed for as climate change continues isn’t the only cause of flooding.
Two other big causes are directly man-made: deforestation and pavement.
Tree roots hold soil, and trees on the whole sequester enormous amounts of water. If you strip a hillside of its trees, the land both dries out (no shade) and becomes prone to mudslides. Downhill and downstream, it’s more prone to flooding.
Pave that land, or any land, and you’ve doubled down. Now the water can’t even soak into the ground. It runs straight into sewers or creeks. The latter either flood themselves or causing flooding downstream. A good example here in Pittsburgh is the river town of Millvale, which was waist-deep in water after storms a few years ago. A primary cause was new subdivisions and parking lots upstream on a creek called Girty’s Run.
Water that runs off pavement carries toxins that spilled, that leak from cars, that oxidize from auto exhaust. Water that runs off big suburban lawns not only runs faster than it did when that land was forest or field; it carries herbicides, too, that can affect marine life hundreds of miles away. (One border of the Chesapeake Bay watershed lies in the middle of Pennsylvania.)
Go to a forest during a rainstorm. You can be there a long time before you even get wet. The canopy catches the drops, slows their descent. Rain that reaches ground level doesn’t yet reach ground. It’s captured in the cups of fallen leaves, say.
Anti-conservationists belittle the saving of wild land as merely a cosmetic favor, but this isn’t just a matter of esthetics. Beauty is immanent in function; the two are one. The leaves and pine needles and mushrooms slow the stormwater further. It seeps to earth only slowly, through decaying matter, and then through layers of soil and subterreanean minerals. Cleansed, it makes its way to streams and rivers.
Marshes are better sponges still. It was only very recently that our civilization began to value marshes as buffer zones, as ecological kidneys. Still, plenty of people continue to think of them as swamps that need a good layer of concrete to fix them up right.
Floods happen in undamaged nature, but they are rarer and less severe.
We have to stop thinking we can improve on how wild land does things, especially things like turning storms into clean water. Otherwise, we’re just telling stormwater where to go, and then shaking our fists when it goes there.