Political borders define countries and set imperatives, constraints and strategies. These human-drawn boundaries are not always natural nor, necessarily, constant. River basins, especially those that cross national or ethnic boundaries, define the history of the regions they traverse and shape their future relationships. 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 River, stretching more than 4,000 kilometers (2,485 miles), is the 10th-longest river in the world. Its major tributaries, the Shilka and Amgun, are fed by smaller rivers emanating from Mongolia and Russia.