the ones that come to mind are:
1. your village could be wiped out by a flood
2. this one is the most important, your village is located at the bank of a river, this means that without any sort of water protection you leave yourselves open and vulnerable to attacks by ship.
3. if you're at a river bank then the village guards are looking at a straight distance, the most advantages land would be higher up where you can see any enemy coming from miles ahead.