This is the 15111th most frequent Polish word.
rzekę
"Rzekę" is the accusative form of "rzeka," meaning "river" in English.
The word 'rzekę' here refers to the direct object of the action of crossing, which is the word for 'river' in the accusative case.
Przeszliśmy przez rzekę, gdy zapadł zmrok.
We crossed the river as night fell.
This sentence does not use 'rzekę' directly but shows the word in its locative form 'rzeki' to explain spatial relation.
Nie dotarliśmy do rzeki z powodu złej pogody.
We did not reach the river due to bad weather.
In this sentence, 'rzekę' is used to highlight its visual perceptibility from a viewpoint, indicating it as the object of observation.