The St Vrain Creek runs from the east side of Rocky Mountain National Park and down through Lyons. The creek starts in Wild Basin, on the southeast quadrant of Rocky Mountain National Park, and flow east to Button Rock Reservoir. Since the creek starts high in the mountains, it runs cold through May and even stays cool in the summer nights. You can expect to find cutthroat trout in the upper reaches and brown trout down before the reservoir. Caddis fly runs May through September.
Note: Use extra care during snow melt runoff which can be until late May or early June. It becomes very dangerous to wade during runoff.
Overall, St Vrain Creek is the 21st most popular fly fishing river of all 47 fly fishing spots in Colorado.