 Population division is initiated either when a river shifts into or forms within the range of a species that cannot cross it, effectively splitting the population in half, or when a small founder group is transported across an existing river through random chance.Usually a river's strength as a barrier is viewed as proportional to its width; wider rivers present a longer crossing distance and thus a greater obstacle to movement.Barrier strength varies within a given river; narrow headwaters are easier to cross than wide downstream channels.