The polluted water in canals indicate the adversities of human involvement in nature and there is need to study human influence on water pollution in canals.
In April this year (2014), the Leeds and Liverpool Canal was closed between Office Lock (2) and Spring Garden Lock (6) following a pollution incident. http://waterwaynews.blogspot.jp/2014/04/pollution-shuts-canal-in-leeds.html
Bohemian Brummie at 11:32:35, 22/12/13 commented on the pollution on the Trent and Mersey Canal just outside the city of Stafford. http://www.walkingforum.co.uk/index.php?topic=25859.0
From time to time, it is best to remove any deposits of silt or debris which buildup in the canals or at the mouth of canals
https://www.youtube.com/watch?v=PACat_tRXgs
https://www.youtube.com/watch?v=m968c8NottM