Orange balsam...

the less aggressive cousin of Himalayan balsam, introduced to Britain from North America in the very early 19th century, and now naturalised along many rivers and canals, including the River Nene. It generally seems to co-exist quite well with the native flora, and I've not seen it do any significant harm in the local area. I quite like the cheery orange flowers, which add a splash of colour when most other species are past their best.