From Steve Schweitzer's A Fly Fishing Guide To Rocky Mountain National Park, flyfishingrmnp.com:
The Big Thompson River in Moraine Park is a rare location in which you can catch a Grand Slam without moving to other waters. However, greenback cutthroat are rare and probably hybridized. ...
Most people access the river from Moraine Park and Fern Lake Trail. From the Beaver Meadows Entrance, take Bear Lake Road to Moraine Park Road to Fern Lake Road to the Fern Lake Trailhead or Moraine Park. Another way to fish Forest Canyon is from the west side of the Park. Follow the Poudre Lake Trail from Milner Pass for just over a mile to the junction of Ute Trail. Take Ute Trail through the pine forest to the pass and travel off trail to the east down to the headwaters of the Big Thompson River. Follow the river into Forest Canyon. Good fishing starts about a mile from the trail. This isn''t a walk in the park, though.