"The Bighead watershed is one of the highest quality watersheds in the Southwestern Region of Ontario. The watershed has good base flow conditions, and much of the watershed is characterized by cool, clear streams which provide good trout habitat. In general, the watercourses are favorably low in plant nutrients; hence there are no aesthetic problems resulting from excessive plant growths. The Rocklyn Creek tributary, Walters Creek tributary, as well as the main Bighead River upstream from Bognor, are of particularly high quality and support excellent invertebrate communities."
I'm sure that some of this high quality is due to the widespread presence of forests and wetlands in the watershed and the relatively low impact of agriculture.