This is the 16455th most frequent Russian word.
ручей
Stream or brook.
The word 'ручей' refers to a small stream of water.
В лесу протекает маленький ручей.
A small brook flows in the forest.
Here, 'ручей' is used to describe the geographical feature over which the bridge is being constructed.
Дети строили мостик через ручей.
The children were building a small bridge over the brook.
In this case, 'ручей' specifies the location of the water being described.