Please take a look at this. It shows what has been happening to the Windrush over many years but is very striking this year after 4 months + of untreated sewage spills from Bourton on the Water and Witney, to name two sources we know a lot about.
Look at the very quickly developing effects that we observed downstream of the spills last winter. they were easily linked as being caused by the sewage. At the same time, look at what the river looks like underwater this April. These shots were taken near Burford.
The results are so similar that you will need a keen eye to spot the difference.