Water lettuce, Pista Stratiotes, has been spotted on the river. This is a non-native invasive species and in warmer waters can present a serious problem. In this country it is on sale for ponds and aquariums, but should not be released into the wild. The leaf rosettes produce stolons that can give rise to daughter rosettes, these can detach from the parent, and enable the species to spread through a waterway.
At present the view of the GB Non-Native Species Secretariat (NNSS) is that water lettuce is unlikely to survive the winter in the UK. That is not to say that climate change, and the possibility of the development of more hardy varieties, could not result in it becoming a problem in the future.
If you discover a plant on the river that you believe might be a serious invasive threat, please check it out on the NNSS website, and follow the advice they give on reporting it.