If a liquid is dark and murky and you can't see through it, it's turbid. Usually used as a criticism — a turbid river is generally a polluted one, but then again a good pint of real ale should be turbid. Go figure.
Not to be confused with turgid, meaning swollen or inflated, though it almost always is. When applied to literary criticism, both words are highly critical and take on slightly different meanings — turgid means "pompous or bombastic" and turbid tends to mean "confused or muddled." If a critic calls you both turbid and turgid, it might be time to think of another career.
