My little book on building dams
Dams are not hard to build at all. Smaller ones are easy, I needed a large one in a gully between two hills.
I did alot of studing on this before having two BOZOS come out and ruin the job. I knew what I wanted and how to build it but I had no dozer, no place here to rent one either. The morons had dozers and supposed to know what they were doing but too stupid to do what I wanted! 10' deep pond, ya right I stand in the lowest part and look over top the dam! I already spent about $600 (or more) for nothing and decided I will wait till I buy my own backhoe or dozer and do it myself. Good place to go 4 wheeling in the jeeps is all I have there!
Anyway to answer your questions.
First the water side does not have to have much taper, do it as you want as long as it does not fall back in, like straight up would fall when wet. It does not hurt to taper this side and does add strength, also make it easier to get out if an aninaml or person falls in (bigger dams) but you get that strength by thickness and does not matter what side it's on.
The back side of the dam needs to be very thick at the base, this is where the most water pressure/weight is. The dam can slope, bank to the top. Keep it thick all the way to the top. Remember that water only wants to go one direction, forward, and to get there it is trying to push the dam over backwards, so the top needs support as well. Taper up to the top and keep it thick. How thick depends on the on the size of the pond, Like someone already said. Shape and width etc.. also come into effect.
For instance it takes a much smaller dam to make a large pond if your only damming up a 2' wide outlet than it would to dam up a 10' wide outlet for the same size pond. In otherwords your only holding back that 2' of water and the natural bank of the land is holding back the other 8' with that small dam, where as with a 10' wide dam your holding it all back with only the dam.
Think of that part like a board or a steel beam. 2' wide a 2x4 may be great for a heavy load, 10' wide it won't support much so you need alot more. Basically same with dams.
When building your dam use the most solid earth you have. Clay is the best. The rest of your pond is going to be natural earth that is already there right, the dam can be also. The more solid the earth will pack the better your dam will be and the smaller it can be also. Top soil good for planting in is probably the worst for a dam! If you use enough it will work also, but what a pain I think.
Packing your dam is about the most important thing there is. And probably where the most people mess up and have a leaking dam also. If using heavy equipment like a dozer on a large dam like mine should have been, then drive over it often, alot! Lay some clay a few feet thick, drive over it and pack tight as posible, build up more, drive more. The better you get this packed the less chance of pinhole leaks and larger leaks too. Also the harder it is packed the harder it is to dig into, many critters may choose a softer location if it is too tight.
"I've built a 3 foot wide, 1.5 ft tall, about 8" thick dam."
So here's what I would do for a great dam.
Your building a small 3' dam, if you can drive a riding mower or 4 wheel drive jeep, car, or what ever you have over the dam alot it helps!! Toss on a layer of clay, all the way across the width and length, drive over it about 4-6 times packing it all, toss on more clay drive etc.. The heavier the vehicle the thicker the layer can be, but the thinner each layer the tighter it packs, so your choice on that. Also moist but not wet clay would be the best for packing. Think of packing a mold to press into a brick. Dry clay will crumble, soupy clay would squirt out under pressure, moist clay pack tight and stays. About the same for a dam.
Since your only going 1 1/2' high I would do 3 6" layers and drive a car over it well each layer. I would go for about 2'-3' thick dam at the base at least and 1' at the top. The thicker the better but that should work for a small one.
Also I would go about 6" higher than I want the water if you have any running water into that area that will rise the pond sometimes. Make a small scoop into the top like laying in a 6" pipe, or better yet actaully lay in a small section of pipe. This will prevent the top of the dam from washing away durring high water levels if it over flows. It just runs through the pipe and no errosion.
To seal your pond or dam plain old wood ash from the wood burner is great. Dump it into the water at the dam so it just spreads out like a dust fog in the water. This lets the water carry the tiny ash particals into any tiny leaks that you may have. It may not work for a 1/2" tunnel of a crawdad very well, but it might if the water runs slow enough, but anyway it works great for tiny leaks you may have now and helps seal future ones as they occure sometimes too like at the base where it settles eventually. If you think latter you have a tiny leak again, toss in another bucket of ash.
For people building a compleate new pond, pigs are great for packing the new pond tight also, pen them up and let them run the pond. Those tiny little hoofed feet and heavy weight really does some good packing. Dig a new pond and run pigs all year till the get too market hog size and your pond should not leak, if it ever does leak, run hogs again
DUCKS, those are good for sealling ponds and dams also. Grab a duck by the neck and beat the ground down tight 
Duh, no that's not right, DON'T DO THAT, it was a joke!
Ducks are good seriously though. Most people don't know a duck will try to preserve it's habbitat and can find those little leaks. Sometimes when you see ducks diving under they may be feeding, but alot of the time they swim to the bottom and pack a little leak to trying to stop it, it works. If you like ducks and have a pond it's a good combination.
Well that's my little book on building dams. I have built smaller dams for both streams to create small ponds, and normally standing water to rise the levels with rain runoff. Only problem I have really had I could not fix is that big 10' deep pond I wanted and only because I have no equipment to do it myself. That's MORE than a ton of dirt, Big area! Maybe 50' -70' or more acrossed the dam, I never messured it, but I think I could sit my 70' trailer house on top of it. For sure we make nice loose circles driving the jeep in that area, round and round and round. Well at least I don't have to worry about running over any fish since I don't have any
