�y��vlog

They manipulated the model to see if it could produce the same branching patterns, and it soon identified a specific ratio between the forces of incision and soil creep that acted like a tipping point or switch.

One would expect incision to spread indefinitely if unchecked, but a process called soil creep smoothes over the land, filling up the cracks with a slow, yet steady, drift of soil.

Neither region is strongly influenced by the faults and folds of a tectonic boundary, Perron says, allowing the team to compare incision and soil creep without too much interference from other variables.

After mapping the river networks in each region, the team created a mathematical model that included equations for soil creep and incision in river channels surrounded by raised mountain ridges.

The intricate networks of tributaries in California's Salinas Valley, for example, indicate that incision is winning out over soil creep, he says—a sign of the softer rock and higher levels of runoff in the region, compared with Pennsylvania's older, harder rocks and higher infiltration of water into the soil.