unnavigable

adjective3

1. •
   Not navigable: difficult or impossible to navigate.
2. •
   Impossible to sail through; impassible to watercraft, unusable as a waterway (for example, too shallow).

   • A road journey of 70 miles past unnavigable rapids leads to Juba, from which it is seven days' steaming down river through to Kosti, on the Sudan Railways. — 1950
3. •
   That is totally impassible: cannot be used for any reason.

   • The trail was unnavigable since the flood washed it away.