What is meant by topographic barrier?
Whether something is considered a topographic barrier for an organism depends on the species being considered and its ability to traverse the terrain from one side of the barrier to the other. There's no blanket classification of what is considered a topographic barrier and what isn't. 

For example, a certain mountain range may be a topographic gradient to a salamander (due to limited food, extreme weather conditions, etc.), but these same mountains might not be for a brown bear.