Until walking along the Cromford Canal I'd thought that duckweed could be only one colour, i.e. green. But here it was red! The photos don't do it justice: it looked a lot redder than this, like a freshly laid cycle lane (a red one, obviously). The duckweed had collected between two narrows, just in this section between High Peak Junction and Whatstandwell.
A new species of Duckweed (Lemna) was discovered in the wild in Britain during 2007 - this is the Red Duckweed (Red Duckweed (Lemna turionifolia Landolt). It was discovered at two separate sites.
It is described in the most recent edition of Watsonia (Vol 27, Part 2) and was an article published by R. Lansdown. In brief, Lansdown states that Red Duckweed was discovered in Dorset in May 2007 then Lincolnshire in July of the same year. It is a native of North America (yet another botanical species to become established from those shores) and northern Asia. Lansdown suggests that this species is overlooked in Britain.
A couple of branches made this "hole" in the duckweed: you can see that the bottom of the canal here is very close to the top.
This is the more usual colour of duckweed, seen here on the Shroppie near Ellesmere Port in 2005.