any pond, pool, ditch, gutter or watercourse which is so foul or in such a state as to be prejudicial to health or a nuisance;

any part of a watercourse, not being a part ordinarily navigated by vessels employed in the carriage of goods by water, which is so choked or silted up as to obstruct or impede the proper flow of water and thereby to cause a nuisance, or give rise to conditions prejudicial to health:

Provided that in the case of an alleged nuisance under paragraph (b) nothing in this subsection shall be deemed to impose any liability on any person other than the person by whose act of default the nuisance arises or continues.