After the death of his paddling companion, a German shepherd-labrador retriever mix named Jasper, Kim Trevathan began a series of solitary upstream kayaking quests in search of what he calls "liminal zones," transitional areas where dammed reservoirs give way to the current of the rivers that feed them. For four years he scoured the rivers and lakes of America, where environmentally damaging, and now decaying, man-made structures have transformed...