Movement:
* Stagnant: This suggests a lack of movement or flow.
* Still: This refers to a lack of movement, often associated with water.
* Stopped: This implies that the flow has been halted.
* Blocked: This indicates something is preventing the flow.
Other:
* Stiff: This could be used to describe something that is not flowing smoothly.
* Constrained: This suggests a lack of freedom or natural movement.
* Restricted: This implies that the flow is limited or controlled.
Example:
* The river was flowing freely, but then it became stagnant after the dam was built.
To determine the best antonym, consider the specific meaning of "flowing" in the sentence you are trying to write.