Which of the following is an example of a non-perennial water source?

A river that dries up in the summer is classified as a non-perennial water source because it does not consistently flow throughout the year. Non-perennial water sources are characterized by their intermittent availability; they can experience periods of drought where water is absent, particularly during certain seasons.

In contrast, the other choices represent perennial sources or those that are consistently available. A lake that exists year-round and a reservoir with a constant water level are both examples of perennial sources, as they maintain water levels throughout different seasons. Likewise, an underground aquifer can be considered a perennial source because it can provide a continuous supply of water over time, despite fluctuations in recharge rates. Thus, the distinction lies in the reliability of water presence throughout the year, making the river that dries up in the summer the correct identification of a non-perennial water source.