Wes, a great topic for a number of reasons. For one, simply a vastly under utilized time when compared to prime time on the Snake, June through October. And yet, fish are eating and some great days on the river can be had.
As opposed to the Snake River up above the reservoir, the stream below the dam is 3-5F warmer, more of an effect due to the level of the dam outlet as opposed to effect of springs. Therefore the low is around the mid 30s, and, by the first week of March, the river is already warming up to 40-41F and the spawn is on for Rainbow.
Food is ample from midges to scuds to stoneflies to dace and water bugs. This gives the angler many options to try. Alewives, a bait fish in Merritt, often suffer a kill at ice-off and flood the river below providing a protein jump of biblical proportions, lol.
Nymphing is a tried & true method. Fish are apt to be in deep water, conserving energy and waiting for something to come close to their mouth. So, well delivered successive casts, working the lanes, is required. Or, you may find trout at the edge of high water, using habitat. As hatches do happen, usually midges, there might be opportunity for some dry flies. Midwestern salmonflies (stoneflies) are a staple above the falls and hatch late March-early April. A weighted black or brown stonefly nymph, or weighted girdle bug, can be a winner. The biggest female salmonfly I have encountered is 2.5 inches long. Typically, males are 1-1.5 inches.
Streamers are good. While wooly buggers are a staple, I tend to use bigger saltwater flies anywhere from 2-4 inches long fished slow and covering all ground. Mostly, just a drift. As opposed to any stripping, I use a hand curl to bring a streamer across or up and over habitat, very slow. I also “nymph” them, dead drifting them trough riffle-run-pool to imitate dead alewives or dace and has been successful in that method. The fly, like a white zonker or other bait fish pattern. It is not unusual for a trout to stalk your fly, whether streamer or nymph, so be xtra aware on the end up swing and just before your fly gets to the surface. If the fish taps/hits it and does not hook at that time, immediately lower you rod, letting the fly drop and you may get another whack.
Another strategy for early winter is an egg pattern. As the browns spawn, rainbows are scarfing up eggs on the downstream drift. One just has to be careful about fishing on the redds, or stepping on them.