Good ideas and comment so far. With a lake (dam) that size you need professional engineering help - trust me you don't want a failed dam. If it is to be in Utah then

Contact Mike Otto
[email protected]
http://www.ottosdirtservice.com

Mike knows about dry dirt and dams


and Mike Mitchell -
Queen of the River Consultants from Longmont, Colorado his number is (303)651-2514

for ideas.