From Steve Schweitzer's A Fly Fishing Guide To Rocky Mountain National Park, flyfishingrmnp.com:
Lower Timber Creek can be followed from Route 34 at the Timber Creek rest area upstream for a steep three quarters of a mile. Expect to find smaller brook trout in abundance in this section. ...
Timber Lake Trailhead on US-34 north of Grand Lake to Timber Lake Trail. The deceptive first three-quarters of a mile lures into thinking of an easy hike, however, the next mile brings some of the steepest yet most beautiful scenery on the trail. If you are in decent shape, this 4.4 mile (one way) day hike will bring you to some fine stream fishing for cutthroat trout. For some, the trail is just long and steep enough to consider making this a one night camp over. The trail gets plenty of day use, but only for the first mile or so. The beauty of the first mile of the trail is marred by the sounds of traffic on nearby Trail Ridge Road. Continuing onward take your through some of the most scenic sections of the trail and away from nearby traffic sounds. The trail skirts Jackstraw Mountain and, at the junction of Long Meadows Trail, follows Timber Creek to Timber Lake. There are plenty of camping sites, five to be exact, that put you a hop-skip-jump of fine stream fishing.