Do you want to make this?

Dams are awesome in scenario making as a piece of eye-candy. Making a dam can be a nice way of making a river crossing, and not only sticking to shallows and just a simple bridge. Making a dam is quite simple, and done well it can often make players go "wow".

1. First of all, you start off with a blank map. (no duh)

Boring? Don't worry, it will get better.

2. After that, you add some cliff. You need decent height difference to make a good dam.

It doesn't have to be rectangular or very symetrical, just plob down some cliffs. Use the cliff tool to do that. Or, elevation increasing without cliff works as well.

3. Ok, after that is done, you put some water under the cliff, since a dam is...to control water flow. So it makes perfect sense to put some water underneath. You can also put some on the top. (I forgot and I put it later)

Depending on your theme, put your water accordingly. For example, snowy and cold should use the Norse river.

4. Now use the copy elevation and make a straight sheer drop from your cliff. If you don't make it straight, it would be difficult to get your wall material to line up easily.

Mine is on a diagonal, making it horizonal is easier. Since horizontal would not have the jagged shape. But anyways, continue on.

5. Now you choose your main building material. This will be the main thing people will see on your dam. Since this is an Atlantean Dam I am making, I put some Atlantean wall. Just plob some down.

You can also use Trojan walls, normal walls with upgrades. (Citadel walls, Fortified walls, Iron walls..etc).

6. Now put your walls onto the cliff. Make sure they line up right and doesn't show any terrain.

If you somehow can't cover a terrain up, trying jamming another wall in, put columns and such, boulder walls, or simply painting the terrain a colour that is close to your dam color.

7. Here I finally added the top water. (You can do this earlier) I added more cliff to make it more realistic.

8. To make the dam more realistic, you must have an outlet to let the water leave. Use shrunken waterfalls. (K to expand, Shift-K to shrink) Put walls to line up the waterfalls. The walls should be upgraded later. Also, i added some Atlantean Connectors in the side to make it seems more connecting to the cliff.

9. The dam is almost done. The basics are there. Now we would put would decorations in. Here I added trees, mists, a Poseidon Statue, 2 gaia wonders (Shrunk), enlarged columns, and some other less-significants.

Be creative. This is the part that can make your dam from normal looking to a well-done piece of eye-candy.

After that is done, you have your dam!

To make a nice screenie/cinematic with the dam, remember to add several things.

1. Change your view! Put it at a different angle. Zoom also. This can make your eye-candy better than it's viewed from above. Use A+Z, Page Down+Page up for maximum effect.
2. Render Sky. Unless, its night, no one like to look at an ugly black sky. Also make sure turn off see-through and make sure you turn off fog and black map. Add snow and rain as neccesary.
3. Add sun/moon/star, animals, units. This will make it look lively. I added shrunk mirror towers to make the sun. Also some seagulls in the background.
4. Lightling. Makes your whole area looks different, as shown below: