Jeb
20000 gal per day is a lot of water you should see that if it was going threw the dam.
rapid water loss when full then slowing down as the depth of the water goes down is standard.
a shovel is a good way to start digging holes, hand held post hole diggers will get you down 3 feet. Some hand held augers in the right conditions will go 10 feet, A back hoe is the easiest but now you are spending money.
Otto
Dig behind the dam along the bottom to see how much if any water is passing under the dam.This will tell where the water is going either under the dam or straight down.
