Yes you will need a permit.
A person using a suction dredge in Mt. needs to get a permit from The Dept. of Enviromental Quality, 1520 East Sixth Ave., P.O. Box 200901, Helena Mt. 59620-0901. (406) 444-3080
If the suction dredge intake is less than 4 inches in diameter, a general permit is required. If the intake is larger than 4 inches, a more complicated permit is required, for which there is a significant charge.
The Mt. Dept. of Natural Resources and Conservation, and the US Army Corps of Engineers may also require permits.
You may want to contact the Montana Bureau of Mines and Geology.
Montana Tech
Butte Mt. 59701
(406) 496-4167
Hope this is up to date and helps. But, I heard that you could get the permit from Montana Fish, Wildlife, and Parks, but not absolutly sure.