Why do waterfalls never dry up?

Waterfalls normally don't dry up because the stream that feeds them doesn't dry up. However, some of the waterfalls in the Alligator Rivers region of the Northern Territory of Australia such as the Jim Jim Falls and the Twin Falls have been known to dry up during the dry season and start again in the wet season as the creeks that feed them resume running.The water would be able to erode rocks that it passes over when it comes to a cliff,causing the riverbed to be lowered and it creates a canyon upstream for the water to go back up to the waterfall.