I was just thinking to myself what an etymological curiosity the spelling of the word yeah is. (Can "etymological" refer to spellings?) I would expect a word such as yeah to have a simpler, more straightforward spelling, being both recent and informal, and therefore not having a long written history.
Two possibilities come to mind: a) the spelling is derived from the old English word yea; b) there is simply no easy alternative (ye? yeh?).