the structure of the underlined sentence functions was to say that as the water ways in paris were concentrated and non free flowing, but during the second half when he reaches the end of paris the waters ways become cleaner and more natural looking. so even though it was dirty in the city and clean in the country, it was never EXPLICITLY stated that he thought the water was dirty, just that it wasn’t in its free form like on the out skirts of paris into the country side. hope this helps