Evolution of river systems
By Sid Perkins
A river’s erosion downward and across a landscape is based on a variety of factors, including terrain steepness and the arrangement of tributaries.
In the March 7 Science , geophysicist Sean Willett of ETH Zurich and colleagues boiled down these factors into a single parameter called χ (the Greek letter chi), which typically becomes larger as distance from a river’s mouth increases. The scientists then calculated values for and mapped it for various river networks, including the southeastern United States.