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.
Rivers all run into a larger body of water.
They all have the potential for pollution.
Rivers all change the land even if they are not fast moving.
Rivers provide life for organisms, even stagnant polluted rivers.
Rivers have a beginning and an end.
Rivers can flood.
Rivers are part of the water cycle.
River systems in the world are very different. Some are already polluted while others are at risk of pollution. Some are abundant in life while others only support the basic organisms.