From The River's Edge:
7/28 THE GALLATIN IS CURRENTLY DIRTY FROM RAINS IN THE PARK, CALL OR STOP IN FOR THE LATEST UPDATE* Dry fly or dry-dropper fishing is the way to go up in the Gallatin Canyon. If you are wading under or around spruce tree's- try a spruce moth. For the rest of the canyon expect to see several types of caddis, yellow sally stones and pmd's. Between the canyon and YNP expect to see a bit of the same so take slightly smaller imitations of the above mentioned patterns. Due to the warmer temps, the lower reaches of the Gallatin from I-90 to Three Forks is closed to fishing from 2PM til Midnight. To be honest, the river is very mossy down there so I'd recommend giving those fish a break and avoid the lower river all together. Have fun and release fish as quickly as possible, it's hot outside!