Although this stamp is from China, the horses made me think of Mongolia. And sure enough, it turns out this river is in Inner Mongolia.
Outer Mongolia is what we now know as simply Mongolia. Inner Mongolia is an autonomous region of northern China.
I was all set to use this stamp for R, but when I looked into finding the name, it turned out that this winding river is called Xilinguole, in the Xilinguole Grassland National Nature Reserve which covers an area of 10,786 square kilometers. The name Xilinguole means river on hilly lands in Mongolian. (though it looks pretty flat in this view)
find more X at Sunday Stamps