Heat is a killer. A hard lesson here is: conyoneering is emphatically not mountaineering. In any group of mountaineers, the number of potential summiteers shrinks as the mountain lets people know just how hard it is to gain altitude solely via one's own power. In other words, mountains often weed out the unfit so early in the game that, once they realize they have bitten off more than they can chew, they can often return fairly easily downhill to their staging zone. While descending most canyon trails, the ease and coolness of the descent are seductive. It's a breeze even for the unfit or the unprepared, until the time comes to hike back up. Then when it's all too often a hot, dry, hard, agonizing, and often torturous physiological contrast to the descent, the unfit get weeded out late in the game and get weeded out brutally, sometimes fatally.