From Steve Schweitzer's A Fly Fishing Guide To Rocky Mountain National Park, flyfishingrmnp.com:
9500 ft down to Chasm Falls
For about a mile and a quarter above Chasm Falls, the Fall River contains sparse populations of trout. The best way to access the water is to follow Old Fall River Rad to Chasm Falls and find a pull-out parking space at the ...
Getting there: Horseshoe Park proper extends from the western point of Endovalley campground eastward through Hoseshoe Park to Aspenglen campground on the eastern end of the valley. Essentially, there is no formal hiking required as most of the stream can be accessed via roadside pull-offs. Access the river by the Fall River Entrance station to the Park and via Endovalley Road, which leads to the one-way Old Fall River Road.