Rivers and creeks are good because they carry thing for long distances. They can carry boulders with relative ease. Native Americans lived and hunted near water. All that living and hunting near them puts all their living things like arrowheads near them. They get buried or shot into a creek or whatever. A little water will erode and wash those points right into it and carry it for a while. It dumps them right where it dumps cobbles and gravel. We used to do a lot of digging for them on property we had access to. We would dig and move a lot of dirt and find a bunch of points but the river will do that work for you. When you dig you get them as they were dropped discarded or lost. But the river will carry them from private property to public.