Word for something that cannot be traversed because it is too deep

Solution 1:

The river was unfordable

unfordable

: incapable of being forded : impassable an unfordable river

M-W

We could not cross the river. The river was unfordable.

Solution 2:

Impassable sounds right to me.

http://www.dictionary.com/browse/impassable?s=t

The first definition fits :

not passable; not allowing passage over, through, along, etc.:

Solution 3:

Impassable describes the fact you can't get across it, though it doesn't explain why.

Solution 4:

Unfathomable can be used in this context

Unfathomable:

Difficult or impossible to measure: the unfathomable depths