Answer:
...a force acts upon it.
Explanation:
"An object in motion tends to stay in motion unless a force acts upon it."
This same rule implies to running water. Water typically runs from a higher place to a lower place with the usage of gravity (except for certain rare cases). Until there is something blocking (for example, a dam, or a let down in the steepness of the drop), the water will continue to move forward.
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