From Steve Schweitzer's A Fly Fishing Guide To Rocky Mountain National Park, flyfishingrmnp.com:
From Cache La Poudre River
Although not specifically along the Poudre River or along any trail, the creek deserves mention for its quality brook trout and brown trout fishing. Access is best from the ...
Mirror Lake Trail is one of the most remote trails in the Park. Access to the trail iseither from Corral Creek Trail to the west or from Mummy Pass Trail outside the Park to the northeast. Travel time from metropolitan Denver area is over three hours - so simply due to the travel time, you'll find fewer trail mates along the way. The short jaunt of 1.4 miles to Mirror Lake is quite serene as it passes through a tall stand of pines. The trail becomes root-filled and narrow as it is less traveled than others in the area. About a third of the way up you'll begin to hear Cascade Creek rumbling down from Mirror Lake.