From Steve Schweitzer's A Fly Fishing Guide To Rocky Mountain National Park, flyfishingrmnp.com:
Onahu Creek (to 10,500 ft)
Essentially the first three miles of the creek is considered drive-by brookie heaven. Due to its ease of access and short trail distance, this is a popular trail amongst anglers. In the spring, the creek runs full, feeding ...
Onahu Creek Trailhead on US-34 north of Grand Lake to Onahu Creek Trail. From the trailhead, the creek is close at hand for a majority of the way up this short two to three mile fishing journey. The creek and trail are in the heart of west slope moose country, so don't be surprised if one greets you on the trail or stream along the way. The trail is well maintained and is a popular day hike destination for fishers and non-fishers alike. A great full day fishing 7.3 miles loop begins at Onahu Creek trailhead, fishing your way upstream to the beautiful little meadow about two miles up. Hike through the meadow to meet a trail spur at the eastern edge of the meadows, hiking out to the Tonahutu Creek trailhead. If fishing with a buddy, leave a car at both trailheads to avoid a half mile roadside hike back to Onahu Creek trailhead.