North Fork of the Flathead River in Glacier National Park Professional Review and Guide
"The mystery of the North Fork, and the Middle Fork for that matter, is when and what type of fish you will catch on a given stretch of river. Biologists and anglers alike realize that the North Fork benefits from the lakes connected to it, especially Flathead Lake. Fish that migrate upstream from Flathead Lake help make it possible to catch big fish in the river.
After fishing most of the river and at random enough times, you should be able to catch whitefish, a few rainbows, and smaller cutthroat in the river year-round. However, it may be a lot of hard fishing for a meager fare from November to May. In the summer the primary fare is cutthroats between 8 and 13 inches long. You can also catch larger cutthroat and rainbows up to 20 inches long. Species include: westslope cutthroat trout, mountain whitefish, cutthroat-rainbow hybrid, rainbow trout, bull trout, lake trout, and Yellowstone cutthroat trout."