US river rising in NW Wyoming that enters the Missouri near the North Dakota border (11)

I believe the answer is:

yellowstone

(Other definitions for yellowstone that I've seen before include "US river" , "River with falls in Wyoming" , "First US national park, 1872" , "Topaz, perhaps" .)

I've seen this clue in The Mirror.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.