I once did a module at Uni to do with Remote Sensing but I can't quite remember. It indeed depends on what sort of sensors they used. Could be an indication of altitude or just algae. Judging by what's at the end of the river Avon it's probably the actual contents of the water as oppose to altitude.
It's a bloom of algae caused by agricultural and sewage runoff and probably exacerbated by the water shortage in the SE of England.
Yeah, human waste ... and agricultural runoff. That's where the Thames empties into the sea and the Thames is where sewage gets discharged into after treatment. London of course, straddles the Thames and has a population of around 7.5 million but that's just one city. There are large population densities all the way up from London as well, all needing somewhere to get rid of their waste. The agricultural runoff is where the fertilisers used in farming, and laden with Nitrogen, Phosphorous and Potassium and other carbon-based chemicals (called organic because they are carbon based), get washed into rivers by rain. Algae feeds on these organic chemicals which is why you get an algal bloom.
that is disgusting Rob! Did you go swimming? If you did, do you glow or are you growing ferns out of your ears?
That looks like paradise compared to the river Don in Sheffield, South Yorkshire. Here, nobody dare look at the water, nevermind swimming in it!