A river crossing on the way to Trepina gorge. I found rivers in Central Australia rather fascinating. They flow on top of the water-table layer which can be fairly deep. On top of the water-table is sand and loose rocks that water can freely seep and flow through. So the rivers flow above the surface only after good rains. The rest of the time water flows under ground and the riverbeds look bone dry. The river gum trees that grow along the banks of rivers have a very deep root system that is able to reach water flowing under ground. It is a very strange sight - huge healthy trees growing around rivers and creeks that seemingly have not seen water in years. MacDonnell Ranges, Northern Territory, Australia.