All the larger streams that feed into the Bitterroot River harbor rainbow, cutthroat, and brook trout, and some of the higher lakes in the Bitterroots, such as the Big Creek Lakes, are known for good fishing.
For the angler, though, the real news is the Bitterroot. The river seems largely untainted by the effects of a century’s worth of forestry, farming, suburban sprawl, and irrigation. In fact, even in the busy heart of the valley, the cottonwoods and willows that line the shore shield the angler from the realities of Bitterroot development.
The trout here are both numerous and large. Rainbows and browns fill the majority of creeks, but cutthroat and the elusive bull are also apprehended.
The lower part of the river near Missoula [1] is where the lunkers are most likely lurking (and, rumor has it, largemouth bass), while farther south, up Connor way, is where the trout are thickest.
The state has established 10 fishing-access sites on the river, and there’s also easy access from bridges. At no place is the Bitterroot far from the road, although remember to ask for permission before crossing private land.
Boat rentals are available in Hamilton [2], and organized float and fishing trips are offered by the region’s many outfitters.
Links:
[1] http://www.moon.com/destinations/montana/missoula-and-northwestern-montana/missoula
[2] http://www.moon.com/destinations/montana/missoula-and-northwestern-montana/the-bitterroot-valley/the-lower-bitterroot/hamilton