The Old Dillon Reservoir Trail
is located between Dillon and Frisco on the north side of the Dillon Dam Road. The trail begins at this trailhead, climbs through a switchback and tops out on a ridge between the old and new reservoirs.
The downhill segment of the trail can be difficult to find. Ride clockwise around the reservoir, looking for an or doubletrack that fades below the dam. Follow this east staying below the dam. At the 11 O'clock position on the north side of the reservoir, just east of the dirt access road look for an old, nondescript doubletrack descending north.
The trail descends parallel to Interstate 70 and is quite fun, with a series of drainage bar whoops. Shortly after passing around a gate, look for a singletrack forking to the right with a "trail" marker post. Take this east to S. Adams Ave. near the base of the dam. The last section of the descent is very steep and loose. It's rideable, but a little hairy.