The Missoula area rivers have large populations of the so called "salmon fly", really a large stonefly with an orange belly of the Pteronarcys genus. The nymphs of these flies are large (#4) when mature and black in color. These stoneflies hatch progressively upriver from lower elevations to higher elevations. The beginning of spring runoff means these flies will begin hatching in the next few weeks.
Therefore, one of the most effective flies to use during spring runoff in your local rivers (all of them have the "salmonfly") is to toss large #4 black stonelfy imitations (Brook's Stone, Bitch Creek, Montana Stone, Kaufmann's Black Stone, etc.) on sinking or sinktip lines. Cast the fly 45 degress upstream and let it swing downstream until it is directly below you and near shore. Be ready for a fish to pick up the fly at anytime during the swing, including just as your ready to cast it back upstream again. During the 12 years I lived in Montana, I had some very fine days fishing the high, discolored water of spring runoff with large black stonefly nymphs.