According to the wiki (http://wiki.openstreetmap.org/wiki/WikiProject_Thailand), “Khlong” should be tagged with waterway=canal. During my holidays in the South, I saw that rivers are generally called Khlong (and many small streams also!), only a small minority of rivers are called Mae Nam (e.g. Trang river). There are also huay, which is typically used for streams - but some streams are also called khlong, and I found also a khlong huay…
Some examples for streams/rivers which are called khlong:
- Khlong Salakhun http://www.panoramio.com/photo/64512201
- Khlong Sok http://www.panoramio.com/photo/64507694
- Khlong Bang Muang http://www.panoramio.com/photo/64505863
- Khlong Hin Dan http://www.panoramio.com/photo/64506809
I think the wiki needs to be updated.