![]() ![]() They aren’t really affected by single storm events. In contrast, large watersheds flood in response to widespread and sustained wet weather. ![]() Usually, this water is already long gone by the time the next storm comes. Smaller watersheds only flood during single intense storm events, called flash floods. This seems simple, but it is a real conceptual challenge in understanding flooding, not to mention trying to control it. But, if everywhere in the basin is having an unseasonably wet year, that can add up into major flooding as all that water concentrates into a single waterway. A big rainstorm in one city is not going to make a dent in the total flow of this river. At the complete opposite end of the spectrum, more than a million square miles (or three million square kilometers) make up the drainage area of the Mississippi River in the U.S. If it happens to be raining hard directly on your house, the gutter will flood, maybe even overtop the road onto the sidewalk. ![]() Your front yard is a watershed to the gutter on the street. The further you go, the larger the watershed becomes as more and more streams contribute to the drainage. And as you move downstream, those funnels start to combine. A watershed is the entire area of land where precipitation collects and drains into a common outlet it’s a funnel. The topography of the land and the long-term geologic processes mean that streams join and concentrate the further you move downstream just like the branches of a tree. Part of the reason for that confusion, I think, is the huge variety of scales we have when talking about flooding. But, I think there’s still uncertainty in how floods actually occur. We all know generally what a flood is: too much water in one place at one time. Today, we’re talking about how large scale flood control structures work on rivers. We still can’t change how much it rains (at least in the short term), but we’ve found lots of ways to manage that water once it reaches the earth to limit the danger it poses to lives and property. Luckily, we’ve developed strategies and structures over the years to reduce our vulnerability and mitigate our risk. Nearly every major city across the world is susceptible to extreme rainfall and has areas that are vulnerable to flood risk. And if you haven’t, be careful in thinking it can’t happen to you. If you’ve ever experienced one yourself, you know how powerless it feels to be up against mother nature. Every year floods make their way through populated areas, costing lives and millions of dollars in damages, devastating communities, and grinding local economies to a halt. ![]()
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