Final answer:
A perennial stream flows year-round, while an ephemeral stream flows only in response to precipitation and may dry up for part of the year.
Step-by-step explanation:
A perennial stream or river is one that flows year-round, whereas a ephemeral stream does not flow the entire year. A perennial stream receives enough water from sources like groundwater seepage to maintain a flow throughout the year, even during periods of low precipitation.
On the other hand, an ephemeral stream only flows in direct response to precipitation such as rainfall, and it may dry up for parts of the year, particularly in dry climates or during periods of drought.