Dredging creeks/rivers/streams I'd like to think would stay banned, leave them for the hobbyists or semi pro prospectors, hydraulic sluicing got banned that long ago you'd be struggling to find a coloured picture of it in action, although I have seen a coloured YouTube clip of a water monitor in action then there is also what they get up to in the Klondike, but they're mining claims, a completely different subject to licensed prospecting
Had a friend some years back ask me to build him a dredge, after a lengthy discussion on the do's and don'ts of legal prospecting I ended up lending him some dive gear, shovel bucket and a high banker, and man did the lad do well, although he paid no attention to my advice on altitude diving and bottom times
I'm all for leaving our creeks/rivers and streams to prospectors with pans/sieves/sluices/bankers/trommels, if anyone did have a mining lease with permission to dredge it I would love to see it in action, however I could imagine the size of your rehab plan, to put it back the way it was would equal a pretty hefty diesel bill.
Finally, a 4" (100mm) suction dredge is not small, the raping something that sized would do to a creek would be outright shocking, and I'd like to know how one would intend on backfilling :|