Leaving a fountain in the pond over the winter can have several consequences. Even if the pump is turned off there is still water in and around the pump. The water will freeze and expand, putting the working parts at risk for breaking. Some fountain owners will choose to leave their fountain on with the thought that moving water won't freeze. This is not a guarantee. If temperatures get extremely cold the ice will start to form on the pump and with prolonged extreme temperatures the entire fountain will freeze. This puts great stress on the pump because the motor is working overtime resulting in the motor burning up. Fountains that are left on during the winter can create great works of art as the ice begins to form and build on itself. Towering ice mountains will appear.