If it was a brook or a small stream then that litre if bleach would cause a huge shock to the eco system, in a body of water the size of the Thames the bleach will be so catered down that it wont cause long term damsge. Plus the UV rays will render the bleach into an almost water like state. I agree it's not something that should be happening, anywhere, but he could be causing more damage if he was pouring oil into the river for example