This is question copy-pasted from a forum. I thought you might have something interesting to say about it:
"I've been thinking why do some rivers have dolphins while other rivers lack them?
The Indus, Ganges, Amazon and Yangtze all recently had or have river dolphins, but the Mississippi, Nile, Yellow and the Congo lack them.
I know the Congo has some rather nasty rapids near it's mouth, but it otherwise seems perfect. The Nile might be too narrow but the Yellow and Mississippi seem fine."