The first major question is how/what are you looking to fish.
In a river I suggest going with three lines. Number one is a designated floating line, which is the most versatile and useful line in terms of presentation techniques.
Next I would use a slime, clear intermediate sinking line to get the fly just subsurface Also works good in frog water.
Finally I would go with a sink tip fly line. The floating section "pulls" the tip up on the retrieve, rising the fly up as well in the process (before it sinks again). They also allow you to avoid boulders/submerged objects you might hang up on as only the tip sinks.
Now if you wanted one line, I suggest a rio versitip style of line. You get a main floating line woth a floating tip and a various selection of different tips for different depths and scenarios.
For example my 8 # rig features a floating line, sink tip, and I have a slime line. On a river like the stave, the slime line works well in areas lile the old boat launch or just downstream past the corner. The floating line can be used for nymphing, streamers, and dry flies while the sink tip covers the deep streamer fishing.