There are a number of good reasons to use Non Return Valves (often referred to as a Check Valve) in a pond system set up..
Quite simply a non return valve only allows water to flow in one direction and automatically prevents water back flowing (reverse flow) when water in the pipework reverses direction. This may happen in the event of an unexpected power cut say or even just turning your equipment off for general maintenance. They are one of the few self-automated valves that do not need any manual operation to work efficiently.
There are two main types for ponds – Spring Loaded and Swing Type.