BRIGG STILL GREEN BUT WHY?

What some people may see as an unwanted feature of the River Ancholme in Brigg town centre is still with us - one month into autumn 2020.
From late summer, greenery starts to appear and spread over the water - rather spoiling the view for passers-by on the County Bridge and those using the river-facing benches along the towpath. 

 

The covering has usually disappeared long before now. But on October 20 much was still evident and may even linger a bit longer.
Can anyone say why 2020 has seen such a prolonged stay by the green stuff?
 

In Octobers past which were Coronavirus free, Ancholme Rowing Club has staged its annual Head races - attracting crews from across the north and midlands. We've posted many photographs of competitors setting off to take part in the Head further along the river and later returning to the boathouse on Manley Gardens but can't find any that show abundant green material present on the surface.
We've included a few examples below to demonstrate what we mean.
Sadly, the green covering does tend to retain litter discarded by uncaring members of the public.