Why do oceans have salt water but rivers have fresh water?
The water in rivers comes from rain. Rainwater does not have any salt in it. The river picks up a small amount of salt from the ground as it travels downhill. The salt enters the ocean at the river's mouth. The saltwater mixture becomes saltier as water evaporates.
Instruction: Why do oceans have salt water but rivers have fresh water?
Context: 
Response: The water in rivers comes from rain. Rainwater does not have any salt in it. The river picks up a small amount of salt from the ground as it travels downhill. The salt enters the ocean at the river's mouth. The saltwater mixture becomes saltier as water evaporates.