Litter in water courses is not solely a result of people throwing their rubbish into the water. Rubbish can blow out of bins, or fall off of vehicles. And then there’s garden debris, often caused by ill-placed furniture or compost heaps.
Rivers always look worse after heavy rain. This is because the river becomes more turbulent, deeper, and flows faster than before. Rubbish which was previously hidden or dispersed can all congregate in one location, forming a raft of litter.
Fly-tipping as in the picture above is sometimes a problem. This picture is from Pan Streams, and the culprit was caught and offered the choice of prosecution or clearing it out. Well done Isle of Wight Council! It can be difficult to establish where and how to report fly-tipping and this is a useful guide.