For a river running perpendicular to the trench line it is obvious that one cannot dig a trench through water, but then there was little danger of the enemy infantry attacking through water – those that didn’t drown would make very slow progress through a very narrow gap and thus pose little problem on anything other than a minor tactical scale.

Thus, trenches were dug as close to the banks of the river as practicable, and then usually doubling back through a 90 degree turn to enfilade the river. Barbed wire obstacles could be pushed right up to the river back, or even into the river itself.

You can get an idea of what this looked like from a trench map