I bet those rivers have had (trash) thrown into them for more than 150 years. Here in the US our heritage only goes back a few hundred years. So other than items discarded over the last few centuries, the most we have to encounter may be fossils, Indian artifacts, etc.
But in th UK, you guys have a time line that dates back for thousands of years. I know you will find some more interesting artifacts to show us.
Welcome to the forum!
Robert