Rivers have frequently made convenient borders between neighboring groups. But what happens when years and years of erosion and shifting weather patterns cause a river to change its course? What happens when someone builds a dam or irrigation controls that alter the river’s path? It had been a nearly universally-practiced convention that with the gradual and natural changes to a river over the course of many years, the boundary also gradually moves, but that sudden or man-made alterations would not change the location of a border. But what happens when…
