A check valve is used to ensure airflow or waterflow only travels one way. A check valve on a submersible pond pump or sump pump prevents water from flowing back through the pump if power is lost or the pump turns off. This will prevent unwanted drainage especially important in pondless waterfalls or multi-tiered fountains where waterfall catchment basins hold large quantities of water. The check valve spring or flap door will shut and seal if ever the pump turns off. In airlines and aeration systems a check valve protects the air compressor by ensure that no water flows into the electrical componants of the pump if the air pressure stops. Check valves should also be used at the junction between diffusers and airline in aeration systems to prevent water from entering the airline of the pump stops.