Changing the Course of Rivers and History

Punjab means “five rivers.” The region in northern Pakistan is named for the great rivers that branch through the landscape, creating an ancient cradle of civilization and a modern agricultural breadbasket. But two geologists have found strong evidence that it was not always so.

The Sutlej, Ravi, Chennab, and Jellum Rivers flow westward to join the Indus River and drain into the Arabian Sea, west of Pakistan. But the rivers once flowed eastward into the Ganges River and drained into the Bay of Bengal, east of India, the scientists say. The rivers were rerouted 5 million years ago, probably as the ongoing collision of the Indian and Eurasian tectonic plates bulldozed the terrain, tilting it westward or lifting up river-diverting mountains.

To reconstruct this momentous continental and historical shift, Peter Clift of the United Kingdom’s University of Aberdeen and Jerzy Blusztajn of Woods Hole Oceanographic Institution found telltale clues beneath the ocean—in the Indus Fan, an apron of sediments pumped onto the seafloor over millennia by the mighty Indus River (which gave India its name). The fan is 6 miles (10 kilometers) thick and extends about 900 miles (1,500 kilometers) into the Arabian Sea.

The scientists analyzed seismic surveys of the fan’s sedimentary layers. They found that the layers started to accumulate twice as fast sometime after about 5 million years ago—indicating a large and geologically sudden increase of the Indus outflow.

Clift and Blusztajn also analyzed sediment samples cored from the seafloor and found that the sediments’ isotopic compositions switched dramatically 5 million years ago. Before then, the relatively high ratios of neodymium-143 to neodymium-144 in the sediments indicated that they came from the Karakoram Mountains, far north of Punjab, and were eroded from the old southern edge of the Eurasian Plate. After 5 million years ago, the neodymium ratio was much lower, indicating that the sediments were eroded from the front edge of the Indian Plate and came from the Himalayan Mountains to the east—the source of the four modern Punjabi rivers that now flow into the Indus. The scientists concluded that the rivers were once Ganges tributaries that were “captured” by the Indus River.

“This is the first time such a major sediment capture event has been dated,” Blusztajn said. It shows the potential “to use ancient sediments to reconstruct what mountains looked like in the past.”

The scientists reported their findings in the Dec. 15, 2005, issue of Nature, which called the research “an extraordinary piece of detective work.