Watersheds

Leon and Michael are camping, and it has just started raining. Falling raindrops hit the tent and roll down outside on the tent walls. Let’s take a closer look at one of the tent walls. Here, some raindrops run together and form tiny trickles that drip onto the ground. Then, the trickles of rainwater flow along the ground and create a tiny stream.

This stream finally reaches the nearby river. If we follow the river, we see that other streams join and make the river bigger and bigger. As the water flows along, some of it will evaporate and some seep into the ground, but most water will keep flowing, until it eventually ends up in one place - in this case the sea. There are also other rivers that drain into the same sea. The area of land from which all water flows into this sea, is a watershed of this sea.

A big watershed is made up of smaller sub-watersheds. So, every river has its own watershed - the area of land containing all small streams feeding into that river. And each stream has its own, even smaller, watershed. How do we know what watershed a piece of land belongs to? Well, water always flows downhill.

It moves from a higher point to a lower one, along the easiest possible path, because of gravity. So gravity and the shape of land, topography, determine the path water takes and the place where water ends up. If you look at the boys’ tent again, you see it has two sloping sides that meet at the top. Raindrops falling onto one side of the tent, drain to the left. And raindrops falling on the other side, end up running down on the right.

And it’s the same thing with a hill, a slope, or a mountain ridge. If we connected all the highest points in an area, the resulting line would be the boundary, or divide, that separates one watershed from another. Since water flows downwards, whatever happens in the upper part of a watershed, affects what happens in the lower part of the watershed. For example, the amount of rainfall up in the mountains will influence the amount of water in a lake. And it’s not just rain!

All sorts of things from the watershed end up in rivers and streams. Things are carried along by the water and finally reach a lake or the sea. For example, chemical runoff from a field might cause water pollution even far away from the source of pollution. That’s why it’s important to know how watersheds work and what happens in them, so that we can manage our water resources better. Why are you digging a ditch next to the tent, Leon?!

I’m steering our watershed. The ditch will redirect all the rainwater to the river, so it won’t flood the tent!