What’s the oldest river in the world? Well, Larapinta in western Australia only has water for a few days each year, but it has probably been around for four hundred million years.
How do you work out the age of a river, anyway? It is based on one convenient fact: rivers tend to meander (curve back and forth) only on flat plains. If you find a river that meanders through a mountain, you can assume that the mountain is younger than the river.
The technical term for these features are water gaps: a mountain range rises up around an existing river, and the river erodes it down to form a valley-like bisection. The Kali Gandaki Gorge, for example, cuts through the Himalayas because it is older than the Himalayas. That’s not difficult, by the way. The Himalayas are a very young mountain range.
Larapinta, also known as the Finke River, is surrounded by mountains that sprung up four hundred million years ago during the Alice Springs Orogeny, the same tectonic event that (probably) turned Uluru on its side. So we can infer that the river is older than that event. And, by this measure, there are no older rivers that still flow. It’s not irrefutable evidence, but it’s probably the best we’ll find.