North St Vrain Creek begins at Thunder Lake which is a 6.8 mile hike in the Wild Basin area of Rocky Mountain National Park. The trailhead begins on the south end of the parking area at the Wild Basin Ranger Station and is a moderate climb for the first half. The last two miles is a steeper up-hill climb, so save some energy for the last push. Even into September the cutthroats were eager to take an ant pattern on the surface and it was fun to see them take it.
Three miles into the hike towards Thunder Lake is the Bluebird Lake trailhead (12.6 miles round trip). This trail follows Ouzel creek to both Ouzel and Bluebird lake. Both Ouzel creek and lake offer good fishing for cutthroats but Bluebird Lake is barren.
The hike to Ouzel Lake is 1.9 miles and the trailhead is past the falls from the parking area. I ran out of season this year but it is one of the first places I plan to hit next summer.
Another area to explore is Cony Creek above Calypso Falls. The falls are 2.3 miles up the Thunder Lake trail but the trail that follows the creek goes to Pear Lake. With several camping areas this would also be a nice area to stay in.
The river below Calypso Falls is prime pocket water but gets considerably more traffic being less then a mile up river. If you want some easy access for cuttrout on the north st vrain creek, this would be an easy place to get to.
One angler I talked to on the way down from Thunder Lake said "it felt like there was a fish in every pocket eager to take my fly".
Remember all the water in Rocky Mountain National Park is catch and release and you need to crimp down your barb.
Below Horse Creek bridge the water is artificial flies and lures only down to Button Rock Reservoir. The reservoir is owned by the City of Longmont and passes are $10 a day. I need to explore this area more to find out where you can get access because some of it is private land.
The river has good access after the reservoir along CR 80 until it runs into Highway 36. The river continues into Lyons where it merges with the south fork to become the St Vrain River.
Here is a video of Thunder Lake during a trip in September and you can also check out other pictures from the trip on Facebook/nocoflyfishing