The Geological Crime Scene
In the highlands of the Serranía de Cuenca, a dramatic geological event took place: one river stole the headwaters of another. This process, known as stream piracy or river capture, occurred when the Júcar River eroded backwards and intercepted the upper reaches of a neighboring river, diverting its flow into the Júcar's own channel. The theft happened from below, through a network of underground passages dissolved in the region's soluble rock.
The landscape here is a classic example of karst topography. The bedrock is predominantly limestone and dolomite from the Cretaceous period, roughly 90 million years old. Over millennia, rainwater, which is naturally slightly acidic, seeped into fractures in the rock, dissolving it and creating a subterranean plumbing system of conduits and caves. This underground network became the scene of the crime. The Júcar River, flowing at a lower elevation and with a steeper gradient, had more erosive power. Its waters exploited the underground karst system, carving a path beneath the watershed divide and eventually tapping into the flow of the other river from below.
Evidence of the Capture
Geologists can read the evidence of this piracy written on the landscape itself. The most prominent clue is the "elbow of capture," a sharp, almost 90-degree turn the Júcar River makes near the town of Villalba de la Sierra. This is the point where the captured waters were rerouted into the Júcar's course.
The spectacular canyon of the Júcar River is another piece of evidence. Boosted by the stolen water, the Júcar's flow increased dramatically, allowing it to carve the deep, impressive gorge visible today from viewpoints like the Ventano del Diablo ("Devil's Window"). This viewpoint, carved into the limestone, shows the river's erosional power as it winds through a canyon that reaches depths of around 200 meters.
Meanwhile, the beheaded river was left diminished. Its former valley, now containing a much smaller stream than the one that carved it, is known as an underfit stream—a classic sign of river capture. The same geological processes are responsible for the nearby "Ciudad Encantada" (Enchanted City), a natural site where water and wind have sculpted the Cretaceous limestone and dolomite into fantastic shapes resembling mushrooms, animals, and bridges. This whole area is a large-scale outdoor site of karst geology in action.