The army slowly advanced across the river.
I have to admit that I would like to use the imperfect avanzaba here, given that the advance was slow. 
Addition: por el río is more likely “along the river”; there are a number of other translations to that effect. Surely “a través del río” would be less ambiguous?
Imperfect: The army was advancing slowly along the river. (on-going action)
Preterite: The army advanced slowly along the river (completed action)
The slowness of the advance is the same in both examples. In this particular case, the choice of tense depends of whether the writer is describing an on-going or completed action.
I think the idea of a completed action would be better expressed using a verb like atravesar, but I get your point.
If the sentence is the answer to “¿Qué pasó durante la noche?”, I have no problem.