I have found that optimizing the use of spoil behind the dam is rare. For my first pond, I asked the experienced builder to put all the extra excavated dirt behind the dam. He said it would not fit, and later said he was wrong so he moved it twice to get it there (honest guy). For my second pond, I asked the same but got a very steep back on the dam and piles of dirt spread all around the pond (first pond builder had retired and that's what I got the second time around). I built my 3rd pond myself and still did not put as much dirt behind the dam as I should have. I think the amount that will go behind a dam to make a gentle slope is deceiving. Hopefully, any more ponds that I build will use the spoil better behind the dam. It not only makes for a gentle slope that looks good, but also adds strength to the dam.