Fly across the United States, and you'll notice that the rounded hills and stream-carved valleys passing below are often evenly spaced, like fingers on a hand. Now researchers have found that a surprisingly simple formula governs how those patterns emerge. Their paper represents an "elegant answer" to the valley-spacing puzzle, says geoscientist Richard Alley of Pennsylvania State University, University Park.
Geologist Taylor Perron of the Massachusetts Institute of Technology in Cambridge and colleagues have spent several years unraveling the mystery of evenly spaced valleys. "[It's] one of the most widespread and striking patterns on Earth's surface," he says. "And whenever you see a pattern in nature, there's a good chance that pattern is telling you something important about the system that produced it."
That something, it turns out, is the tension between two landscaping forces: the tendency of water runoff to carve streambeds and excavate valleys, and the tendency of soil to creep downhill, thereby rounding off hilltops and filling in those valleys. So, for example, if two streams start off too close together, the faster one will eventually win out because the other will not receive enough water to keep up with soil creep. Conversely, if two streams start out very far apart, water has an easier time carving out a new stream between them. The net effect is to ensure even spacing between streams--and thus hills, the team reports tomorrow in Nature. "It was like disassembling a complicated machine only to discover that it's controlled by a single knob," Perron says.
One critical factor in the formula is rainfall, but in a surprising way, Perron explains. He says the team expected that in locations with more rain, the stream-carving effect would be stronger, effectively bringing the valleys closer together. But the researchers found the opposite. "The more rain a site receives, the wider the valley spacing," says Perron. This is probably because rain speeds up soil creep by stirring the soil and promoting vegetation growth. That "encourages the rain to travel through the soil instead of flowing over the surface and causing erosion," he says.