the big stuff will get cleared by the authorities sharpish as it will block bridges etc and risk more flooding. smaller stuff might get cleared one day.
On this occasion it wont be so much of an issue of dimwits littering. Much of this stuff will have been in people’s houses, shops, businesses, drains, bins, farms and been ripped out and dragged downstream in the flood. if you do volunteer to clean it up wash your hands after.