This is the 2232nd most frequent Polish word.
rzeką
The Polish word "rzeką" means "a river" (instrumental case).
In this sentence, 'rzeką' denotes the instrumental case, implying the location of the action relative to the river.
Idziemy spacerować nad rzeką.
We are going for a walk by the river.
Here, 'rzeką' is used metaphorically to refer to a river-like structure made of sand, depicting creativity.
Chłopiec bawił się rzeką z piasku.
The boy was playing with the sand river.
This usage of 'rzeką' implies an interaction or an attribute metaphorically associated with the river, in instrumental case context.