From Steve Schweitzer's A Fly Fishing Guide To Rocky Mountain National Park, flyfishingrmnp.com:
Wild Basin Trail
This is the mainstay fishing destination on this trail. The further up the creek you travel, the more brook trout and an occasional hybrid cutthroat can be found. Brown trout dominate the first mile of stream beyond the trailhead. ...
From CO-7, take Wild Basin Rd (Co Hwy 15) which ends at the trailhead. Wild Basin Trail takes the hiking fishing to several small streams for an easy day hike to all. just minutes into the hike, the trail crosses Sandbeach Creek and then follows the North St. Vrain closely for about 1.4 miles. Wild Basin Trail takes a sharp turn to the south about 1.5 miles from the trailhead, at the junction of the Campsite Shortcut Trail. It then turns northwesterly, following Ouzel Creek for a short time. The trail is wide and well-maintained for the first mile, then becomes thinned and more rock-strewn as it gains altitude modestly and steadily throughout, While it follows the North St. Vrain, it is flanked by tall pines and aspen, making a beautiful stroll in the autumn as the aspens leaves turn yellow.