。
The Kiso River is often called the Rhine of Japan.
Literal
Kiso-river [topic-は] often Japan [possessive-の] Rhine-river [quotative-と] is-being-called.
~と呼ばれている is the standard 'is (commonly) called X' frame — passive of 呼ぶ ('to call') + ~ている for the ongoing/customary state. しばしば ('often, frequently') is a written-style adverb common in essays and journalism. The comparison to the Rhine is a stock geographical analogy in Japanese textbooks.