I read in a previous article that to much clay in the soil caused cracking. I also read that gray clay is the best because it compacts and holds water the best. The clay I have has started to crack on the top for about 3" in depth. If I'm adding more dirt to the dam, should I plow the top layer and repack before I add the dirt or will the new dirt fill in the gaps and provide a seal. I am thinking about buying a water pump and saturated the ground every couple of days until the dam is built to prevent cracking. Any suggestions?

Too much clay can cause cracking. Clay expands and contracts with moisture, or lack of it. Soil scientists, dam builders and earthmovers will tell you to have 30-40% clay content in a dam. Less than that, soils tend to leak. More than that, cracking becomes a problem. So, mix your soils accordingly.