This is the 8209th most frequent German word.
isar
The Isar is a river in Germany and Austria.
Here, 'Isar' refers to the name of the river Isar, which is the subject of the sentence.
Die Isar fließt durch München.
The Isar flows through Munich.
In this sentence, 'Isar' is again the river's name, specifically mentioning its banks as the location for the picnic.
Wir haben ein Picknick am Ufer der Isar gemacht.
We had a picnic on the banks of the Isar.
'Isar' represents the geographical feature here, emphasizing its popularity with visitors.