The month of October is big fish time in Yellowstone National Park. There will be some warm days but cold, sub-freezing temperatures will be common. During the month you can expect water temperatures to vary from the preferred range for trout to the freezing level depending on the location and elevation of the stream. Cold air temperatures, especially in the morning, are typical. Snow showers will exist throughout the park on occasions. You will start seeing fewer mayflies, and caddisflies, and stoneflies and terrestrial insects just about want exist. The number of streams in the park you can fish with good opportunity becomes less and less as the month goes by.
Yellowstone River
The Grand and Black Canyons of the Yellowstone River can be a good option. While there are no big migratory fish to pursue, Cutthroat trout are plentiful. The river will fish well during warmer days. Sculpin streamers and Blue Winged Olives usually work good.
Gardner River
The river is also a good October choice due to its excellent population of resident trout with a few larger, migratory fish mixed in. These migratory fish move upstream out of the Yellowstone River and are present October. Sculpin streamers give you a chance at one of the bigger fish. Good hatches of Blue Winged Olives take place, especially below the Boiling River. The Boiling River warms the water fishing below can be effective, especially on cold days.
Madison River
The Madison River will still be the most popular river in the park and rightly so. There will be a lot of anglers fishing the runs of pre-spawn fish migrating from Hebgen Lake. Good numbers of resident rainbows and brown trout will be caught throughout its entire length in the park. There will be good hatches of Blue-winged olives, October caddis and some Spotted sedge caddis, Little Sister caddis and White Miller caddis will still be present at the first of the month. Terrestrials including hoppers, ants and beetles won’t exist.
Firehole River
The Firehole River will be a very good choice for the fly angler. The geysers will keep much of the water warm enough for the trout to be feeding and active. It should turn out good numbers of trout throughout the day. Fish running out of Hebgen Lake can access to the lower mile or so of the Firehole River before they are blocked by waterfall. Above the Firehole Falls, the river provides good dry fly fishing. On the warmer days, Blue Winged Olives will be hatching along with White Miller Caddis. October Caddis and some Spotted Sedges will be present.
Gibbon River
Migrating fish from Hebgen Lake will also have access to the lower section of the Gibbon River below Gibbon Falls. This gives the angler about five miles of water to fish with good expectations. There will be some resident trout available there as well.