Many streams would dry up after a rain if it were not for:

• A. Surface runoff
• B. Groundwater flow
• C. Reservoir storage
• D. Evaporation

Answer: (B)

The correct answer is groundwater flow. Groundwater plays a critical role in maintaining stream flow, especially during dry periods or in the time following a rain event. After rain, some of the water infiltrates into the ground and replenishes aquifers. This stored groundwater gradually flows back into streams and rivers, sustaining them even when rainfall is not occurring. This process is essential for ensuring that many streams do not completely dry up after a rain, as groundwater discharges into these bodies of water, providing a reliable source of flow over time.

Surface runoff, on the other hand, refers to water that moves across the land surface and flows into streams immediately after rain, but it does not contribute to long-term stream flow stability. Reservoir storage pertains to the management of water within man-made structures to regulate supply, and evaporation reflects water loss into the atmosphere rather than contributing to stream continuity. Thus, groundwater flow is the key factor that helps maintain the flow in streams following rain events.