Final answer:
A stream might disappear if it was first flowing over impermeable rocks but then changed to flowing over water.
Step-by-step explanation:
A stream might disappear if it was first flowing over impermeable rocks but then changed to flowing over permeable substrates such as sand or gravel. This phenomenon occurs because water permeates into the ground when it encounters permeable materials, where it can then evaporate (especially if it is near the surface), flow beneath the surface, or be stored for long periods, potentially leaving the original stream bed dry.
Where a stream originates, often referred to as source water, has distinct characteristics - typically being cold, clear, and nutrient-poor. As it travels away from the source, a river or stream's channel widens and the current slows down, allowing for more sedimentation which affects the water's movement and quality.
Importantly, at higher elevations, streams are more likely to flow over impermeable materials, maintaining their surface flow. However, when reaching lower elevations, where permeable substrates are more common, the water is likely to percolate into the ground and potentially disappear from the surface.