Access these hikes from the Kolob Canyons Road [1], which begins at I-15 south of Cedar City [2]. You're likely to have the trail to yourself in this section of the park.
This is an excellent day hike from Kolob Canyons Road. The easy-to-moderate trail begins on the left two miles from the Kolob Canyons Visitor Center and heads upstream into the canyon of the Middle Fork of Taylor Creek. Double Arch Alcove is 2.7 miles from the trailhead; a dry fall 0.2 mile farther blocks the way (water flows over it during spring runoff and after rains). A giant rockfall occurred here in June 1990.
From this trail you can also explore the North Fork of Taylor Creek. A separate trail along the South Fork of Taylor Creek leaves the drive at a bend 3.1 miles from the visitor center, then goes 1.2 miles upstream beneath steep canyon walls.
Kolob Arch vies with Landscape Arch in Arches National Park [3] as the world's longest natural rock span. Differences in measurement techniques have resulted in a controversy regarding which is longer: Kolob Arch's span has been measured variously at 292-310 feet, while Landscape Arch's measures at 291-306 feet. Kolob probably takes the prize because the 310-foot measurement was done with an accurate electronic method.
Kolob's height is 330 feet and its vertical thickness is 80 feet. The arch makes a fine destination for a backpacking trip. Spring and autumn are the best seasons to go; summer temperatures rise above 90°F and winter snows make the trails hard to follow.
You have a choice of two moderately difficult trails. La Verkin Creek Trail begins at Lee Pass (elev. 6,080 feet) on Kolob Canyons Scenic Drive, four miles beyond the visitor center. The trail drops into Timber Creek (intermittent flow), crosses over hills to La Verkin Creek (flows year-round), then turns up side canyons to the arch.
The 14-mile round-trip can be done as a long day trip, but you'll enjoy the best lighting for photos at the arch if you camp in the area and see it the following morning. Carry plenty of water for the return trip; the climb to the trailhead can be hot and tiring.
You can also hike to Kolob Arch on the Hop Valley Trail, reached from Kolob Terrace Road. The Hop Valley Trail is seven miles one-way to Kolob Arch with an elevation drop of 1,050 feet; water is available in Hop Valley and La Verkin Creek. You may have to do some wading in the creek, and the trail crosses private land (don't camp there).
Links:
[1] http://www.moon.com/destinations/utah/zion-and-bryce/zion-national-park/sights/kolob-canyons
[2] http://www.moon.com/destinations/utah/zion-and-bryce/cedar-city
[3] http://www.moon.com/destinations/utah/canyonlands/arches-national-park