Madison River
It's logical to think that the river's origin in the high plateau region of Yellowstone Park, which is notoriously cold, would keep the river cold. Conversely, as the Madison River runs right through the most thermally active region in the United States, the water temperature on most sections of the river is remarkably warm for a high mountain river, often more than seventy degrees in the summer. Also, despite lying in a high, mountainous environment, which usually leads to low nutrient content, the Madison has an amazingly high nutrient level in places, creating great fishing for those willing to take the time and effort to access the river.