This is the 8764th most frequent Japanese word.
木曽
"木曽" (Kiso) refers to a region or historic area in Nagano Prefecture, Japan, known for the Kiso Valley.
In this sentence, '木曽' is used as part of the name '木曽川,' referring to the geographical location and thus used nominatively.
木曽川の景色はとても美しい。
The scenery of Kiso River is very beautiful.
Here, '木曽' is utilized to denote the location regionally known as Kiso, acting as the object of the verb '行って.'
私は木曽に行って山登りをしました。
I went to Kiso and did mountain climbing.
In this usage, '木曽' signifies the historical region or area, forming the possessive phrase '木曽の歴史' pairing with '歴史.'