If the embankment (Dam) is 35,000 CUBIC yards, that means a LOT of $$$$$ Typical excavation can run between $40 and $200 per cubic yard depending on access and terrain...Per hour rates can run between $125 and $250 per piece of heavy equipment running, but quite a few cubic yards can be moved in an hour by a qualified and experienced operator....either way, it will not be inexpensive. Be sure to check requirements as I am pretty sure nearly 75 acre feet will fall under not only state, but also federal jurisdiction/regulations...EPA and Army Corps of Engineers (ACE). If there is a blue line creek in the USGS topo, offsetting stream remediation will almost certainly be required by the EPA....