Gallatin River
The Gallatin River gathers water from Gallatin Lake high in Yellowstone Park and cascades through the sub-Alpine vegetation on its way to divide meadows where it's confluence with Fan Creek, and Bacon Rind Creek, bring it to the mature trout stream that it is known as. The confluence meadows on either side of Highway 191 are fishable and do hold some active fish in the braided and convoluted channels of these three rivers. But it is below Bacon Rind Creek, and The Taylor fork the the Gallatin becomes excellent freestone trout water.
The gradient for most of the upper river is controlled by bedrock, (and in some places by the road,) but the meadow sections have sweeping meanders that have pools as deep as 8' (yes that's EIGHT,) and some areas have quicksand as well. This section has excellent access and there are many pull-outs for the angler that likes to drive and fish in rapid succession.
After about 9-10 miles of meandering meadow, the river begins it's descent into Gallatin Canyon (just a brief ways below Snowflake Springs, and outside of Yellowstone Park.) The fishing in the canyon is varied and interesting. Narrow canyon sections provide rapid runs and riffles with the occasional meander pool. Wider sections with meadows provide slower water with sweeping meanders and deep pools.
At Big Sky, Montana, the West Fork of the Gallatin joins the main river and adds considerable a volume of water. The gradient continues to become gentler and the canyon widens to Gallatin Gateway where the river is deep, relatively slow and occasionally meanders into the broad fluvial basin of the Madison River. The river is especially fishable near the town of Gallatin Gateway either through town by the school (go very slowly here,) or at Upper Rainbow Road.
TAYLOR FORK
Arising high near Monument Mountain the Taylor Fork of the Gallatin is occasionally very productive in its lower stretches. The upper reaches are primarily nursery and spawning water. Very late in the spring and early summer, (if it clears early enough,) the last 1/4 - 1/2 mile hold fish from the Gallatin seeking food washed down from the heights. When conditions are right a fish of 20" should not be a surprise in this very thin water. Sneak and cast techniques will be the most successful tactics under these conditions.