First we take a stick of driftwood and draw a circle in the sand, large enough for all five of us to stand easily inside, with Sawyer, 13, in the center. Each of us takes a compass direction, in stationary orbit, family style, around him. I stand to the north, barefoot, and hold a stone in my hand. It represents earth. Ruby, 11, takes the east. She has a feather, representing air. Eli, 14, to the south, holds a candle to signify fire or sun. Marypat, standing west, cradles a small bottle of water. Day 3 on the Yellowstone River, 85 water miles already in our wake. Sawyer’s birth river. The first time we ran the entire rivercourse, in July of 1992, he was a fetal bud, barely a month along, burgeoning in Marypat’s womb. Now he is 13, and this 20-day, 550-mile journey from the edge of Yellowstone Park to the confluence with the Missouri River is in his honor, marking a transition out of childhood into something else that remains a bit vague. The current rumbles past, cold...