http://www.yachtsentinel.co.uk/Rockies/rockies.htm
This link comes from the blog of a 38 foot yacht called "Sentinel" which sailed from Chicago to Vancouver via Wisconsin, Montana and the Rocky mountains. Deep in the article is a reference to a form of pollution you would not even dream of, how it was identified and sorted etc. It will give you some sort of idea of how vigilant you need to be to deliver seamanship to the Thames and its Estuary