Simplify the following text for a Grade 4 native English-speaking person.
Original: Some dolphin species face an uncertain future , especially some river dolphin species such as the Amazon river dolphin , and the Ganges and Yangtze river dolphin , which are critically or seriously endangered .
Simplified: A dolphin 's nose is on top of its head so the dolphin can easily breathe on the surface of the water . A dolphin 's hearing is better than their sight .