https://streamstats.usgs.gov/ss/
In case you haven’t already seen it … here’s an interesting USGS website that will draw the watershed draining to a point picked on a stream. So if the water quality is bad at the point of the Saint’s southernmost 67 crossing, pick that spot on the map and the website will show you the area that drains to that point. If the water quality is good at Gruner Ford Access you could pick the point there and it will show you that watershed. Subtract the second watershed from the first and you should have the region in which the source of the mud & suds is located.
Good luck and happy hunting.