This is a sign at Meguro River with the standard warnings.
All pretty standard stuff except number 2 - "Dont throw rubbish, stones or *dead animals* in the river.
It seems like the throw-dead-animal-in-river thing happens so much that they actually ask people not to do it. Done a search on the subject and seems like it is a popular way to dispose of pets - could be to do with the cost of disposal which varies by area.
Folks who dont want to throw their pet in the river or ask the local ward to "dispose" of it can have it cremated at a cost. Below are prices from this cemetery.
Once the animal has been cremated, they will keep pets in the cemetery grounds at a cost of up to 20000 yen per year - they even have a live camera so that previous pet owners can keep an eye on their pets ashes...
The shelves that you see contain glass cabinets where owners place toys and photos together with the ashes.
How are pets disposed of or laid to rest in your region?
BTW, while searching for "dead animals river", I came across this recent news item reporting how 8 dead cats were found in a river - 6 of them without heads...