Sentence:  Most of the Himalayan rivers have been relatively untouched by dams near their <WSD>sources</WSD>. Now the two great Asian powers, India and China, are rushing to harness them as they cut through some of the world's deepest valleys. 
Meaning of the word marked with <WSD> tags:  Spring; fountainhead; wellhead; any collection of water on or under the surface of the ground in which a stream originates.