The Eagle (2.6 mile from Mesa Reservoir to Eagle Trailhead) and Sage Trails (1.8 mile [2.7 for whole Sage/Eagle Loop]) form a loop between this trailhead and the Eagle Trailhead. Heading southward on the Sage Trail you can either take this loop or head westward on the Cobalt Trail below an old smelter of 1930s to 1940s vintage and connect to the Eagle Trail as it heads toward the Foothills Trailhead. <br />
Another option is to pass the Cobalt Trail (0.9 mile) and take the Eagle Trail as it turns up toward the now dry Mesa Reservoir. A loop hike is available in the Mesa Reservoir area by following the Hidden Valley (1.0 mile) and Mesa Reservoir Trails (0.8 mile).
From Boulder, go north on US 36. Continue past Jay Rd. for 1.5 miles, and take a right on a dirt road. The Foothills trailhead is just a little farther on the left. There are several other trailheads too.
http://www.tripleblaze.com/trail.php?c=3&i=1328