NABSQNO 12S-580403-4271299. The Colonnade is a pair of pillar natural arches located above Twomile Canyon, a tributary of the Green River. The arches are shown as "Natural Arch" on the Bowknot Bend, Utah 1988 topographic map. There are a total of 5 arches at this location.
The two pillar natural arches pictured have spans of about 20 feet. The larger has a height of 16 feet, while the smaller has a height of 10 feet. Because the lintels have breadths smaller than the horizontal extents of the openings, these pillar natural arches are considered adult.
Finding these arches can be a challenge. Access is via a long and complicated 4WD trail, followed by an open desert hike of a little over a mile. Even after you get to the correct location on the topo map, they are hard to spot.
Other Pillar Natural Arch examples: