sufficiently knowledgeable, experienced and qualified to lead a group of teenagers to:
1) have river experience sufficient run a 70-mile trip, knowing conditions, contingencies, and emergency action for all potentialities
2) have the knowledge, skill and teaching ability to lead the building of sufficiently strong canoes
3) have the understanding to sufficiently test/evaluate the crafts
4) have the willingness to tell people no and know which kids just can't do this kind of trip.

There are a few who could make this kind of thing happen and fewer yet who have the inclination and willingness to invest the time, but those are the best possible experiences.

It's like Csikszentmihalyi's model of flow theory: The greatest experiences and "flow" occur when difficulty of the activity is met by ability. When ability supercedes difficulty there is boredom. When difficulty exceeds ability there is disaster (like we nearly had here)

The greatest stretching also results in the greatest experience, but you have to become really expert.

I think most of us have our greatest memories of this kind of thing from when we did something great and difficult but had the leadership that could make it happen.