Riverflow is described as 'allows you to assign a two-finger gesture to an arbitrary action'. There are five alternatives to Riverflow for Mac and Android. The best alternative is BetterTouchTool. It's not free, so if you're looking for a free alternative, you could try Fluid Navigation Gestures. Other great apps like Riverflow are Swish (Paid), Jitouch (Paid) and Multitouch (Paid).
BetterTouchTool is a great, feature packed app that allows you to configure many gestures for your Magic Mouse (1 & 2), Macbook Trackpad and Magic Trackpad (1 & 2) and also Mouse Gestures for normal mice.