A river that builds its own landscape
In the upper basin of the Cabriel River, a remarkable geological process is actively shaping the landscape. A series of stepped, natural dams, known as the Hoyos de los Peces, create terraced pools of clear water. These are not man-made structures but travertine dams, a type of limestone formed by the rapid precipitation of calcium carbonate from the river's water.
The process begins with the region's geology, which is dominated by Mesozoic carbonate rocks like limestone and dolomite. Rainwater, slightly acidic from dissolved atmospheric carbon dioxide, percolates through this rock, dissolving calcium carbonate and carrying it away in solution as calcium bicarbonate. When this mineral-rich water emerges and flows downstream, turbulence and interaction with the air cause it to release its dissolved carbon dioxide. This chemical change makes the water less acidic, forcing the calcium carbonate to precipitate out of the solution and solidify as the mineral calcite. This calcite gradually builds the porous, layered dams, creating a landscape-forming process known as travertine deposition.
A living climate archive
The growth of the Hoyos de los Peces dams is a biological and geological one. The surfaces of the dams are coated in biofilms, which are communities of microorganisms like cyanobacteria (blue-green algae), mosses, and diatoms. These organisms directly help the dam-building process. Through photosynthesis, they remove more carbon dioxide from the water, which accelerates the precipitation of calcite onto their own surfaces. This biological activity is so integral that you can often find twigs and leaves with a fresh, hard coating of calcite. The different colors of the dams, from greens to oranges and browns, are caused by the different species of microorganisms living in the water.
This slow, steady accumulation of minerals makes the dams a record of past environmental conditions. Scientific measurements have established a growth rate of about 3 millimeters per year. However, this growth is entirely dependent on consistent water flow. Recent periods of drought, linked to regional climate change, have caused the river's flow to diminish, halting the deposition process entirely. The dams have stopped growing, showing their sensitivity to climatic shifts. By studying the layers within the travertine, scientists can potentially reconstruct past periods of drought and rainfall, using the dams themselves as a multi-century climate archive.