Idaho steelheading is hard work, especially with a fly. When you do hook a fish it is very rewarding. My best day with a fly is 3 fish, and I have had many, many days of none.
Your 2 major choices of locations are the Salmon and the Clearwater Rivers. We fish the Salmon alot more due to my location. It is a 3.5 hour drive and the Clearwater is around 10. If I had my choice I would fish the Clearwater, the South Fork above Stites to be specific. The fish are much larger and more numerous than the Salmon. We try to fish it around the 20th of Feb each year. You can fish it later but really run the risk of the river being blown out from run off. We stay in Orofino.
I am more successful flyfishing in the spring because the fish have made it up the river to smaller water. You can catch them in the large parts, but I have not had near the success. On the Salmon we fish above Challis (stay there) all the way to Stanley. We nymph with egg patterns, my favorite being fifi.
We also sight fish up high with many of the common steelhead patterns. When I find a fish I put a fly infront of it 2 to 3 times and switch to another color until it takes or leaves. They seem to be sensitve to color more than a specific pattern. I DO NOT fish for them if they are on a redd. This is more common up around Stanley. If they have made it this far and spawning I leave them alone.
The runs have been good this year and I am looking forward to fishing in April. As for water conditions I have to talk to friends that have gone up or I call the sport shops. In Challis I call The Bent Rod, and in Orofino, The Riverside Sports Shop. I don't have the #'s with me but they should be easy enough to find.