Stonefly nymphs are present in most rivers year-round, and are an excellent vehicle for adding weight to your nymphing rig.  I’ll often add wraps of heavy wire to the hook shank of my stonefly nymphs to help them drop to where fish are feeding below the heavy current.  If I’m still not able to sink the nymphs quickly enough, I’ll add some split shot.  My nymphs are sorted in my fly box according to size, weight, and pattern, so I always have a range of options to try out. 

If a river has a high and fast flow, I gauge how much weight my rig needs by watching the speed of my line in the water.  When I notice my leader is moving downstream a bit slower than the speed of the current I see on top, I know I’m in good shape, because it means the flies have reached the slower subsurface water, where the fish tend to hang out in high flows.  I find tight lining to be the most effective strategy, because any type of strike indicator is likely going to drag the flies downstream faster than they should be moving along the river’s bottom.