Created when part of the canyon slid into the river, The Slide Rapid, is not much of anything at lower flows and then it transforms into the largest rapid on the river. Many, if not all of the commercial companies on the river will not run it above 20,000 CFS, and subsequently cancel the entire trip. Portaging is not possible and scouting is exceptionally difficult. Of note, the BLM specifically lists 20,000 CFS as not recommended either.
Below 10,000 CFS and you’ll find yourself wondering what the big deal is as there really isn’t much here and most boaters would chalk it up as a run of the mill Class II.