(The relevant picture, in case you desire to track the bizarre connections in my mind.)
One of the things that happens to land when people get overzealous about making a mark is that we clear it. We cut down all the vegetation, some of which has been around for a long time, and most of which is doing more than just clogging up a nice site for a house. Vegetation, from trees to brushes and shrubs to smaller grasses and floor plants, holds earth together. We all know about the problems folks have in LA, for example, when it rains and all the hillsides that have been deforested and degreened dissolve under the deluge, right? That's a problem. So, the plants hold the ground together. They also keep the ground relatively uncompacted.The roots of plants work their way through the soil, making spaces between the particles of dirt and providing places for water to go. So, a denuded slope has problems:
1. It can't stay together.
2. It can't catch and hold water.
3. It's probably getting compacted due to lack of root activity, which makes it harder for new things to settle in and start to fix the problem.
4. It's sad. (Okay, that might be a stretch.)
The fact that land that previously held water no longer does has larger implications for an area than simply the fact that it's a crappy place to build a house if you're going to have a heavy rainy season. Rain that soaks into the ground slowly filters down into the water table, and this provides, in essence, a liquid bank account for everything in an ecosystem that uses water. (Which is pretty much everyone.) Deposits are made to the water table through rain, but when a stretch is denuded, the deposits dry up. Instead, rainwater runs off, carrying a lot of dirt (including the valuable topsoil) into streams, which then get muddy (which isn't good for fish and their aquatic friends) and all the water rushes off into rivers and, eventually, oceans, where they really don't know how to make good use of rich soil.
So, great, here we go, mucking things up again. What should we do? Build anti-wells to make deposits back into the water table? In essence, yes.
A swale is, basically, a ditch that can be built along the contour of the land to hold water and decrease runoff. If you dig a ditch along the contour of the land (that is, with no low spots where the water will pour out (or with carefully planned low spots so the water seeps out relatively slowly)), the water that's running off will get caught and slowed. It hangs out in the swale rather than rushing away to the ocean, and while it's chilling there in the ditch, some of it will seep back into the ground. You can build a series of swales to catch water and deposit it bit by bit over a long slope. This decreases erosion and improves the water system of that section of land. The relevant mantra of our instructor Brock was: "Slow it, spread it, sink it."
This is pretty damned awesome stuff, if you think about it.
But wait, what does that picture have to do with anything? Well! How do you build a swale? You need to be sure that you're digging your ditch on contour, that is, all along a level, because there's not much point in putting up a dam if you don't build it up to the same level all the way around, right? But it's tricky to eyeball this sort of thing, especially if you've got very uneven terrain. So, you need a level. That's where the a-frame comes in. This is a nifty piece of ancient technology where you tie three sticks together in the shape of an A and then hang something heavy (say, a rock) from a string at the top of the A. Then, you calibrate it so you know where level is, and off you go. It's incredibly simple and also, by the way, excellently cool.
And that, my friends, is why gravity works.