Saint Mary River Drainage in Glacier National Park Professional Review and Guide
"From Gunsight Lake to Saint Mary Falls, the river is slow and marshy. At times it holds an occasional rainbow or two, but access can be boggy, fishing can be slow, and the bugs can be tough. The Mirror Pond area of Upper Saint Mary River is willow-mosquito-moose heaven, boggy stillwater, with some riffles and holes. This section is best fished at low water and after a frost. Spin casters may out-fish fly casters on the still stretches. However, there are a few bigger rainbows for those willing to work for them. Luckily the hike is short and access is easy from Gunsight Overlook Trailhead.
The area between Saint Mary Falls and Saint Mary Lake holds large populations of whitefish, but the tricky current makes catching them difficult. It is better suited for baitfishing than for other types of fishing. However, the fly fisher with sinking line and a small bead-head nymph may take a few whitefish, too."