It seems like you are describing a geographical feature involving rivers and mountains from an overhead view. Based on your description:
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Label A would represent the larger flow of water or river that is formed by the joining of smaller flows. This is often referred to as a main river or river channel.
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Label B would represent the smaller flows of water that converge to form the larger flow. These could be identified as tributaries or streams.
So, in summary:
- A: River or main river
- B: Tributaries or streams