There are so many ways in which rivers are similar. It seems likely that your textbook suggests some answers that you are supposed to know. Because surely you don't need to know that, say, rivers are made of water that flow downhill.
Some other similarities that many rivers have:
- Most rivers are or can be used for transportation -- either big ones that can have ships on them or small ones that can just take canoes.
- Rivers are often used for water to irrigate crops.
- Big rivers were the sites of the earliest civilizations.