Yellowstone National Park, September 18th, 2021

Other than the fall mayfly peak on the Firehole River, the big event is the brown trout migration in the Gardiner, Madison (with accompanying rainbows),Lewis, and Snake River drainages. Crowds of anglers will descend on these events as they progress. The best time to enjoy these events is during unsettled weather which tends to reduce crowds and improves fishing. So watch park weather reports and break out that store of foul weather fishing gear if you are considering fishing these migrations.