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

Question

I am looking for an adjective describing something that cannot be traversed because it is too deep.

We could not cross the river. The river was _______

The only word I can think of that comes close is "insurmountable", but this word usually describes something that cannot be crossed because it is too tall, and doesn't seem to fit for a deep river (or deep valley or deep hole, etc).

Answer 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.

Answer 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.:

Answer 3

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

Answer 4

Unfathomable can be used in this context

Unfathomable:

Difficult or impossible to measure: the unfathomable depths

Answer 5

You have to say something like

They had to turn back at the canyon because it seemed bottomless.

Answer 6

You could consider using the noun “depth” in a single sentence:

“We could not ford the river/[cross the valley] because of its depth."

depth
1 a (1) : a deep place in a body of water ‘fish living at great depths’ (2) : a part that is far from the outside or surface ‘the depths of the woods’ (3) : abyss
…
2 a : the perpendicular measurement downward from a surface
…
3 : the quality of being deep

(Merriam-Webster)

In your context and two-sentence example, you could also consider modifying the river’s/valley’s depth instead of the river/valley itself:

“We could not cross the river/valley. Its depth was too great."