adversus

Adversus is two words sharing one spelling. As a preposition it takes the accusative and means "against, in the face of": adversus hostem. As an adjective, adversus, adversa, adversum, it means "facing, opposite", and by extension "hostile, unfavourable", as in res adversae, "misfortune". Both come from the perfect participle of adverto, "turn towards", which is why the same form does both jobs. The variant adversum works identically as a preposition.

Two traps. Adverso flumine is "upstream", not "against the river", and its opposite is secundo flumine, "with the current" - which is also why secundus means both "second" and "favourable". And in the manuscripts u and v are one letter, so the corpus spells it aduersus.