How would one accomplish this? Is it even possible? Flowmaps normally are images that show the flow of traffic for the water but is associated with static terrain. Is there any method that could create a flowmap from the terrain that could be deleted or removed?

do you mean like analyze the terrain height data and create flowmap from that? sounds doable. something like gradient descent perhaps going in steepest direction down from each pixel and then smoothing and encoding those directions into the flowmap. could get some weird vortices i guess with noisy height map but could be a nice start.

Well, for example a voxel or marching cube based world has dynamic terrain that could and would always change. What if I had a river going through but some guy modified the river and made a new bank or riverbed go off in a different direction?