Ledges are flat slab like drops that water flows over. Formed by flat shelf like formations of rock or river bed shelves they often create a steep drop in the flow that causes a pour over hole to form. In many situations a river or rapid may have several ledges creating staircase (step) like rapids with one drop after the next.
The strength of the pour over created behind a ledge can change dramatically depending on the shape and nature of the ledge, depth of the river and amount of flow. See the river hazards section for more advice on pour overs.