Meanings of become unnavigable in English

To facilitate commerce, he attempted to reopen the old canal dug by Seti I. and his son, which had become unnavigable.

The channels of great rivers become unnavigable, their estuaries are choked up, and harbors which once sheltered large navies are shoaled by dangerous sand-bars.

It appears down an otherwise unremarkable Gloucestershire lane, near where the Thames becomes unnavigable.