WWF Interactive Water Cycle App: Free Rivers

This requires a fairly modern phone and operating system, but works very well if you have those. The app is free and downloadable from the WWF section of the App Store.
Point the app at a flat table, and you will see an interactive river.
Click the icons and you can make it rain, and follow through the other aspects of how the river basin works. Looking forward to using this in the classroom, but here it is in my lounge...