Kanji help! The reading of 上る?

Question:

Why does 上る (noboru) use the 上 kanji? I thought the readings for 上 was ue,a, and jou. So how does it form noboru since noboru doesn't have ue,a, or jou?

Answer:

Because 上る is read as "Noboru".
it also read "Agaru".

though 上 itself doesn't have "Nobo" or "Aga" as its readings.
上 can be read as "Nobo" when it is with る like 上る.
上 can be read as "Aga" when it is with る like 上る.

Its the matter what is added to 上.
in this case, る was added so, there is only 2 ways in reading it:
Noboru, Agaru.

* We never read it as Ueru or Jyouru.