I've been looking at fountains all my life but on this particular night it struck me that the water jets stay together in a coherent parabolic shape until they are almost at the top. Then the shape becomes far less defined as the water comes down. One explanation could be that the water contains sections that are fired at different speeds. This would mean that they wouldn't go quite as high and would come down slightly earlier, broadening the beam. Maybe the pump is a series of pulses rather than a continuous push.
