All you can do is to make sure to clean up your own trash, and try to pick up an additional piece or two that you may see along the river when you're out and about.
Personally, I despise these types of A-holes. Their trash really hurts the aesthetics of what should be beautiful rivers. This type of blatant disregard for the environment, and other people's property, is why a lot of great sites to camp are posted "no camping"
If people were more responsible and cleaned up their mess, landowners might not take as much issue with someone camping along the edge of the river, and wouldn't put up 'no trespassing' or 'no camping' signs. But when they come up on a camp site that looks like a bunch of homeless crackheads have been living there, with trash, and even burned tires, I don't blame them for posting their property.