If you're not real experienced at pouring concrete, I'd try to pour one at a time per truck. It NEVER goes as easy as you think it will.
Get your steel for the walls up and tied before and have them tied into the dam. Keep them off the edge of the forms 2" so you can get the mud down past them with the stone in it.
Make a "key way" for the walls to sit on the dam. You can do it easily enough with a 2x4 placed on the top, center, of the dam when you pour it. Pull it out later in the day or even the next day. That will help lock the wall into the dam.
