Choose a location that is in reach of several stocked stream so you can disperse and deal with adverse weather or flows. Or, choose a stream that needs some friends and see if you can bring a few bags of garbage out and make friends of the landowners.
By the way, don't you hate "that guy" who always throws out lofty ideas, but is never around to lend a hand on even less lofty ones? Why am I being "that guy?" Good luck with the event. Make it about the social activities and the stream(s) chosen should not matter: so long as they are stockt.