From Steve Schweitzer's A Fly Fishing Guide To Rocky Mountain National Park, flyfishingrmnp.com:
Below MacGregor Falls
Like the neighboring Cow Creek, Black Canyon Creek is small but mighty. Plenty of micro plunge pools and very small runs provide ample habitat for small stream brook trout. Don't expect large brookies - an eight inch fish is a ...
US-34 to MacGregor Ave to Devil's Gulch Rd to Lumpy Ridge Rd to Lumpy Ridge Trailhead. Lumpy Ridge Trailhead has plenty of parking, but can get full in a hurry due to the popularity of Lumpy Ridge for rock climbing and the close proximity to Estes Park. The best way to access the MacGregor Falls Trail is to follow Black Canyon Trail through the wide open meadow untl you read the mouth of the canyon at the ebndo fo the meadow. In a tall stand of aged pines, you'll reach a Park Service entrance gate with a sign noting you are about to enter into Park land. Go through the gate staying on Park land and immediately traverse off trail to the left (south) just a few hundred feet to Black Canyon Creek and the MacGregor Falls Trail.