Which term describes an erosion feature formed by running water that creates a narrow valley?

The term that describes an erosion feature formed by running water that creates a narrow valley is "ravine." A ravine is typically characterized by steep sides and is formed by the continuous action of water eroding the land. Over time, as the water flows over and through rock and soil, it carves out a narrow valley that often has distinctive sides, making it easily recognizable as a ravine.

In contrast, an estuary is an area where freshwater from rivers meets and mixes with saltwater from the sea, and it typically features a broader area rather than a narrow valley. A fjord is a glacially-formed valley that is usually deeper and wider than a ravine and has steep cliffs along its sides, generally characterized by a body of water at the bottom. A delta, on the other hand, is a landform created at the mouth of a river where it deposits sediment as it flows into a larger body of water, creating a fan-shaped area rather than a narrow valley.

Thus, the definition and formation process distinctly align ravines with the erosion caused by running water, clearly identifying it as the correct answer.