The Missouri River begins as a high plains river at Three Forks, Montana. From the river’s headwaters downstream to Holter dam, the river flows slowly, both as a free-flowing river and as reservoirs. This portion of the river has some excellent trout fishing during the spring and the fall. The fish here are almost all browns, although a few rainbows come up from the lakes that are located on the river. During the summer, whitefish form most of the action. Browns can still be caught, but they mostly become sluggish, or even move to the deep waters of the lakes.
Below Holter Dam, the Missouri becomes a tailwater stream. This is where most people go to fish the Missouri. The cold outflow from Holter Dam creates a habitat where trout can survive well throughout the year. Rainbows are much more common than further upstream, but browns are also present. The fishing remains excellent downstream to Cascade;it fishes decently all the way to Great Falls.