Many willow species have seed that are surrounded by a cottony mass that helps it blow in the wind and float in the water.
Above is a Peachleaf Willow(Salix amygdaloides) loaded with cottony fluff, ready to fly off for a new start. Female trees of the Salix genus are the cause of this, but if you do not want the mess of this, you can use hybrid willows like the Bigfoot Willow that are male and do not seed.