Clarification concerning my above comments:
Another consideration would be to forego a repeater - instead using a net control on top. That person would relay when any mobile-to-mobile direct communications is not reliable.
Regardless: Any equipment on top should be located to achieve the most radio line of sight. This entails it being on the rim opposite where most of the activity takes place below. It would further help if the equipment were placed on the rim where it "juts" into the canyon the most where the (most of the) course is. If you elect shadowing the participants from above, if possible, that should be done from rim opposite (most of) the activity.
If the river is patroled, contact that agency and/or the search and rescue entity to ask how they get communications out of the canyon. Your event should probably coordinate with them anyway.
Mike WB0DZX