Question

of the four river valleys that supported major early civilizations, which one lost one of its major rivers

Answer 1

I'd say Indus Valley. There used to be River Saraswati whose course (bed) has turned to desert now. 
Yellow river is noted for changing course only. The other two (Nile, Shatt-el-Arab being confluence of Euphrates & Tigris) haven't changed course nor got lost.