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. Jointly owned resources create the potential for cooperation or conflict, depending on whether the interests of either side converge or diverge. Water systems shared by neighboring countries, including the Tigris and the Euphrates , the Mekong and the Rio Grande , often foster a similar dynamic. Almost always, one nation holds the advantage, whether because of economic prowess or simply priority access to the water.