The Collaborative International Dictionary of English v.0.48:

Headwater \Head"wa`ter\ (-w[=a]`), n.
   The source and upper part of a stream; -- commonly used in
   the plural; as, the headwaters of the Missouri.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

headwater
    n 1: the source of a river; "the headwaters of the Nile"

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "headwater":
   font, fount, fountain, fountainhead, head, headspring, headstream,
   headwaters, mainspring, mine, quarry, riverhead, source, spring,
   springhead, well, wellhead, wellspring