The Amur River, which delineates a portion of the China-Russia border as it flows from the desolate reaches of Mongolia into the Strait of Tartary, does not often make the same lists as other storied rivers, such as the Nile, the Danube or the Mississippi. But it has played a vital role in the dynamic between two vast nations. The Amur, one of the longest undammed rivers in the world, is navigable along much of its length and serves as a transport artery for agricultural products and raw materials from the immense Siberian territories of Russia. As Moscow focuses east, maintaining its strategic detente with Beijing, there are opportunities for cooperation and trade between the nations, specifically in the areas of hydrocarbons and agriculture. But there is also the potential for conflict: The Chinese and Russian intentions for the river system fundamentally differ. Russia prizes the Amur for the security and...