You do not need to go into that detail to do a rough estimate.You dont' know if it's good or not if you don't have reponses to this 4 questions :
How many planets will have rivers ?
How many rivers do you need on a planet ?
How much time it takes to create a full river ?
How many people will you assign to this task ?
It's easier to judge on the actual team velocity for the delivered product.
So far, the whole of CiG managed to deliver about 1 planet (and its moons) per year of development.
What makes you think they will go any faster, especially now they showed they are still reworking existing ones ?