The Semi-fluid layer moves in circular movements based on the temperature and density of it's particles. When the particles become less dense, the semi-fluid rises. Once it reaches the bottom of the crust, the particles become dense, and go back to the inner core. Once it's at the inner core, the particles temperature heats up again and causes the circular movements to proceed.
The land area that contributes water to a stream is called a watershed/drainage basin
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