Why are 湖 and 山 in geographical names typically read in On-reading but 川 in Kun-reading?


This question is based on my somewhat limited understanding and observation of
places I have been to so I may be falling victim to a bad sample size. If that
is the case, please alert me.

In the couple of years I have now been living in Japan and from the places I
have been to, I have noticed that rivers, lakes and mountains often include
the kanji 川、湖 or 山 in their names. However, it strikes me as odd that rivers
(宇【う】治【じ】川【がわ】、淀川【よどがわ】、鴨川【かもがわ】 and 江【え】戸【ど】川【がわ】) are typically if not
exclusively read using the Kun-reading of 川【かわ】, while it seems to me that
mountains (比【ひ】叡【えい】山【ざん】、富【ふ】士【じ】山【さん】) and especially lakes
(琵【び】琶【わ】湖【こ】、支【し】笏【こつ】湖【こ】) are typically read using the respective kanji’s
On-reading. Is this a general rule and if so, why is it so?

For clarity: I am only focussing on the pronunciation of 川、山 and 湖; not any of
the other kanji that make up the names I have quoted. There are probably
further kanji that serve similar purposes (as in the English-language examples
of Currumbin _Creek_ versus Brisbane _River_ ) whose kanji I have yet to learn
or notice. Feel free to include such ones in your answers.




The likely reason for these names, at least, is because in this context
(actual names of mountains), as part of a toponym, the kanji is being used as
a standard suffix for "mountain". This suffix is read さん or ざん (i.e., using
the 音読み/ _on reading_ , as you stated).

In other instances and contexts, the kanji 山 is read with the kun reading やま,
but this is usually in family names (e.g., 山田 _Yamada_ and 山本 _Yamamoto_ ) and
other compounds. So basically it is pronounced using the on sound when used as
a suffix in a place name, similar to how English speakers often use the prefix
_Mt._ for "mountain" when it's placed before the name of a mountain.

Of course there is always someone wiser than the first answerer, so I'm hoping
someone else can shed more light on it for you, but if I understand your
question, that's the basic answer to my understanding.

