Rivers move a lot of rock when they move quickly because they have a lot of

Adaptation to their environments!

Explanation:

The mice have different colours of fur because of their surroundings. For example, because the beach mouse lives near- well- the beach, it will have lighter coloured fur to blend in with the sand (and they would need to do that to hide from predators, more than likely). That would be really hard to do if they had dark brown fur hahah~

The same goes for the forest mouse, just instead of sand and changing colour to match something light-coloured, it would change to a dark shade of brown to match the dirt and dark (brown-ish) greens in the forest.

I know this was SUPER late, but I hope I helped in some way ^^