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It seems to me you could do something interesting with floating warning devices that only appear when the water's too deep.

Sounds like overkill.

Stick in the ground will do it.

As long as the ground doesn't dip halfway across...

You can see ripples in the waters surface when there’s a hole in the crossing.

At least when the average depth is shallow enough you’d want to cross in a normal vehicle.

If in doubt, walk it first. (Unless you’re in an area with crocodiles, of course.)

If you can’t walk the crossing, then under no circumstances should you drive it.