Tom youll probably get better answers in the freshwater section. But the answer depends on what your fishing for what kinda rod, how heavy is the line, bait or lure, what kinda bait?
on top of what nautiduck stated, enough weight so that your offering skips over the floor of the stream. if you are repeatedly getting snagged, lighten the weight. i see i lot of guys try and hold bottom. that makes for un natural presentation, especially on a stream that is moving.