There are only a few Class C canyons in the Zion area, and Kolob is certainly the most reliable for good water flow. Problem is, most of the time there TOO MUCH agua running through it because Kolob Creek is the outlet to Kolob Reservoir, which the Washington County Water Conservancy District uses control river levels downstream. If the District is releasing water, the risk of drowning in Kolob Creek is high and descents should wait until a safer time.
If you’re thinking about descending Kolob, be sure to call the Conservancy District (435.673.3617) on a weekday to get the release schedule. If the Creek is flowing 5 CFS or less, it should be reasonably safe for canyoneers experienced in Class C conditions to descend the drainage. If you have your sights set on Kolob, but the flow is too high, consider Boundary Canyon, a near neighbor, which often has a safe flow in high water years.