Most rivers are shallow enough that you could wade in a little way. Not the Congo River in Africa! In some parts it is about 720 feet deep. That makes it the deepest river anyone has ever measured.
How deep is 720 feet? Picture the Statue of Liberty, torch and all. You could stack two of them in the river and still not reach the water's surface. Down at the bottom it is so deep that sunlight can't get there, so it stays dark all the time.
Here's the really wild part. Near the end of the river, before it pours into the ocean, the water rushes through huge rapids and deep underwater canyons. The currents are so strong that they work like invisible walls. Some fish just can't get across the river.
Scientists studied one kind of fish called cichlids. They found that the cichlids on one side of the river were different in their genes from cichlids on the other side, even though they looked almost the same. The rough water kept the two groups apart for so long that they started to change in their own ways.
So a river can be deep enough to hide two Statues of Liberty, and wild enough to split fish families in two!
Ask your kid: If you were a fish stuck on one side of a giant river, what would you want to see on the other side?