It depends what the question means when it says 'the changing of the rivers'. I took it as the rivers are changing size and becoming more spread out. In that case, I chose habitats.
The more water there is, the more habitat there is for sea life.
When a population of birds that are separated into a different area, they might find a different kind of bird and mate with them. likely they might get killed because of their way of eating and surviving.
