How does Aristotle or a medieval scholastic commentator like St. Thomas Aquinas explain how one cause can impede the action of another cause? Or, conversely, how does the removal of an impeding cause generate an effect?
For example, consider a dam, which impedes a river's flow of water. Once a gate in the dame is opened, an impediment to the flow of water is removed, and the water gushes through. However, there is nothing in the cause (in this example: "opening a gate" or "lack of gate"*) that is in the effect (in this example: "water gushing through").
*This latter description makes it explicit that it's a privation, and privations are non-beings and thus cannot be causes; cf. De Principiis Naturæ's discussion on privation.
I'm assuming a standard description of the relation between cause and effect which St. Thomas Aquinas frequently expresses like this (Super Sent. lib. 4 d. 1 q. 1 a. 4 qc. 4 co.):
omne agens agit sibi simile, ideo effectus agentis oportet quod aliquo modo sit in agente.
Every agent effects a thing like to itself, so the effect of the agent must be in some mode in the agent.
{English translation from p. 219 of The Thomist article "The Notion of Equivocal Causality in St. Thomas" The Thomist 79 (2015):213-63.}
Clearly, the river causes is the efficient cause of the gushing water. Is the lack of a dam an instrumental cause? Are impeding causes instrumental causes?