BIG RIVER SYSTEMS AND PLATE TECTONICS

The geological characteristics and origins of big river systems - 22 in Eurasia, 15 in the Americas, 8 in Africa, and one in Australia - are the result of plate tectonics, climate, and changes in relative base level. It is these three, but mostly plate tectonics, that have determined the location, size, shape, and orientation of a large watershed, its longevity and much of its river pattern since 4.2 billion years ago. Big river systems are, in broad terms, relatable to the Wilson Cycle of intracontinental rifting--new passive margins--ocean-continental convergence--new direction of continental tilt and occasional inter continental collisions. Thus big river longevity, like global landscape evolution, has the same time scale as tectonic cycles. The break up of Gondwana was responsible for much of the world's present drainage. The closing of the Tethyan Ocean created most of the mountain chains and basins of southern Eurasia from Spain to Vietnam, a distance of some 13,000 km. In North and South America, ocean-to-continent convergence and accretion of microcontinents created the Cordilleran and Andean orogens and much of the drainage of the Americas. A second important result is the number of rivers that came into existence, essentially in their present form, in the Miocene; a few have ancestors traceable back to the Carboniferous and perhaps even earlier. The key to the long-term survival of a large river is location on a long-lived craton or passive margin, persistence of continental tilt, and ample rainfall all without interruption by desertification, continental glaciation, or volcanism. Conversely, orogenies both destroy and create big rivers.