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Related to churn up: churn out
To cause the movement of something in a liquid (such as sediment of some kind) by stirring. A noun or pronoun can be used between "churn" and "up." I stirred the milk and churned up bits of chocolate powder that had settled to the bottom of the glass. Because we're so close to the shore, your oars will likely churn some pebbles up.
churn something up
to stir up a liquid; to mix up material suspended in water. The oars of our boat churned the shallow water up, leaving little clouds of sediment in our wake. The oars churned up the mud.