The southern trailhead for the Boundary Trail #1207 is on Forest Road 4812-570 immediately south of Tanner Lake (NOTE: The lakes and mountain have been known locally as “Tannen.” They should have been named "Tanner" after an early settler in the area. Not all maps reflect this change. "Tannan" is a mis-spelling.)
From the Tanner Lakes area, the trail heads due east up and down (at times very steeply) along the ridge, passing a junction with the Tanner Lake Trail #1243
and the Fehley Gulch Trail #1231
in just over a mile. It continues on to a junction with the Sucker Creek Trail #906
and the Sucker Creek Trail #1237
at Sucker Creek Gap. You can camp at the Sucker Creek Shelter, which is just east of the Sucker Creek Gap off of the Sucker Creek Trail #1237
, but be aware that the area is frequented by bears.
At Sucker Creek Gap, the Boundary Trail #1207 turns north along the Siskiyou Crest and passes along the east side of Swan Mountain six miles from the southern trailhead. You can do a short off-trail hike to the summit of Swan Mountain from where the Boundary Trail #1207 leaves the saddle just south the mountain.
At about 6.5 miles from the southern trailhead, the trail passes a junction with the little used Green Valley Trail #904
, then continues north past the Denman Cabin to Horse Spring (potential water source). Beyond the spring, the Boundary Trail #1207 traverses Elkhorn Prairie before reaching a junction with the Sturgis Fork Trail #903
and the Mt. Elijah Trail #1206
. Taking a side trip to the summit of Mount Elijah on the #1206 is effort well spent.
From its junction with the Sturgis Fork Trail #903
, the trail contours across meadows on the southeast side of Lake Mountain. It is VERY EASY to lose the trail in these meadows! The saddle north of Lake Mountain is the site of Sparlin Camp, a hunting camp used by locals for years. Just below the camp, along the Elk Creek Trail #1230
, is Delmore Healy Spring (water source).
From Sparlin Camp, the trail climbs up and down along the ridge to a saddle just south of Grayback Mountain. This stretch is not overly used and can be hard to follow in places. It was also burned in 2017 by one of the Miller Complex Fires, so be aware of the potential for falling trees and eroded tread.
From this saddle, the Boundary Trail #1207 swings east around the mountain, past Cold Spring (water source) and a junction with the O'Brien Creek Trail #900
, to its end at Windy Gap. The northern trailhead of the #1207 is the O'Brien Trailhead (upper or lower).