Research published in the journal Geology offers new way to think about how permeable rock shapes landscape over geologic time

Rivers usually carve landscapes from the top down, cutting into high ground and expanding drainage divides away from the largest and lowest rivers. But in Southern Germany's Swabian Alb — a limestone plateau perched between the Rhine and Danube river systems — that process appears to have stalled.

In a new study in the journal Geology, David Litwin, an assistant professor in Temple University’s Department of Earth and Environmental Science, and Luca Malatesta of the Helmholtz Centre for Geosciences in Potsdam, Germany and Tohoku University, Japan show why: water falling on the plateau doesn't stay on the surface long enough to erode it.

Instead, it infiltrates into the karstified limestone and travels underground, re-emerging as springs far downhill. The researchers built a computer model that tracks how much of the energy in falling precipitation is "spent" flowing over the land surface versus lost to this underground detour — and found that the more energy that disappears underground, the longer a plateau like the Alb can hold its ground against erosion, even when a much faster-eroding river sits right next door.

The work offers a new, general way to think about how permeable rock shapes landscapes over geologic time — something existing erosion models, which mostly assume rainfall runs off the surface uniformly, tend to miss.

“Applying the same approach across different rock types,” explained Litwin, “we estimate that this "leakiness" can make highly permeable terrain, such as karst, orders of magnitude more resistant to erosion than its rock strength alone would suggest — helping explain why the Swabian Alb has persisted as a topographic barrier even as the nearby Rhine has captured other parts of the Danube's headwaters.”

According to Litwin, because water is already flowing from the Danube side to the Rhine side underground, through springs like the Aach (the largest spring in Germany), the study suggests the surface divide may only shift when slower processes, such as escarpment collapse or a return to colder, less permeable ground conditions, eventually catch up.