Hello!
Let's start:
1. The members on each side of the river with unfavorable traits for their climate will pass away, manage to produce offspring with favorable traits, or try their best to adapt.
2. A flood isolated the rabbits.
3. If the river and climate conditions remain the same as described above, the original species of rabbit will produce offspring that have favorable traits and eventually most of the rabbits will have those traits.
4. Genetic variation allows different genes to form, and since natural selection is the survival of the fittest, those that were born with favorable traits will survive and reproduce.
5. Geographic isolation.
6. A new species could form because there is no guarantee that the rabbits have the traits necessary to survive. If one with them reproduces, then there's a chance that a new species will be created eventually.
I hope I helped! Feel free to message me or leave a comment below if you need me to explain anything.