Sometimes, the bore can appear as a number of small waves (an undular bore) , or as a major, breaking, wave, or a mix of breaking waves at the shallow, high friction banks and undular bore in the deeper middle*. Sometimes the water behind can be much higher than the water upstream, so the wave is like a step. Because of this height difference the flow of the water, which is off the front of the bore under gravity, reverses the general flow of the river, with the river running, essentially, uphill*.