St Mary's Falls is a 3.2 mile out-and-back trail that leads along a creek up to the St. Mary's Falls. The trail does have an incline, but it's not too difficult, and there are plenty of places to stop and cool down in the creek along the way.
Follow the directions to Buckhorn. They share the same trailhead and both start by riding up Gold Camp Road. Follow Gold Camp Road past the entrance to the Buckhorn singletrack. Go around the left turn and up and over the collapsed tunnel. Once on top of the collapsed tunnel, the singletrack climbs to the right.
http://www.tripleblaze.com/trail.php?c=3&i=551