It's the unwritten law of the river that if it comes downstream it belongs to the finder.
It as also standard practice following a flood, to do a downstream patrol as the water level drops, carrying a long pole, this is to push any dead animals back out into the flow so someone else has the job of burying them! :wh
As someone who has managed various waters I have found the following;
Dead bodies of everything.....
mg:....yes really.
Several boats, and a punt which I sold for £120
A 56 gallon drum of yacht varnish, decanted and shared amongst my friends
Assorted tubs, ornaments, benches and seats, some of which I used myself
Dozens of builders planks, still have shelves made from some of these
Several dozen top quality footballs, usually given away to local kids
I've also found, let behind by anglers, rods, nets, bank sticks complete with full sets of Optonics, umpteen bait boxes, coats, gloves but no hats (?) all of which I returned to the angler concerned if they fished again (yes truly) all except for some of the bait boxes which I still have.
At present there a zillions of tennis balls coming down the river I fish, the keeper had over 50 in less than a week and they are still coming.....???
My least favourite 'left behind' was/is always anglers litter.
I used to collect every scrap, then add quite a bit more to it and keep it in a bag for when the angler visited again.
I would then hand it back to them with a "you brought it now you take it away"
That did the trick, every time, without fail!