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Which river forms most of Inner Mongolia’s border with Russia?

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Wide autumn valley with golden grass in the foreground and low, tree-covered mountains of the Greater Khingan Range in the distance under a clear sky. — Inner Mongolia Da Hinggan (Greater Khingan) Range, southeast of Hailar, Inner Mongolia Autonomous Region, China.

The Argun River (also known as the Ergun River) forms most of the border between Inner Mongolia and Russia.12

Rising in the Greater Khingan Range as the Hailar River, the Argun flows for approximately 1,007 miles, with about 600 miles of its course serving as the international boundary. Near Luoguhe, it joins the Shilka River to form the Amur River.12