A self-constructing landscape
The sixteen terraced lakes at Plitvice are separated by natural dams of travertine, a porous limestone rock. These barriers are not static geological features; they are biologically active structures that grow at a rate of about 1 centimeter per year. This ongoing process of "biomineralization" means the park's park is in a state of slow-motion construction, actively building upon itself.
The process begins as water flows through the region's karst bedrock of limestone and dolomite, becoming supersaturated with calcium bicarbonate. As this mineral-rich water tumbles over obstacles, it aerates. This turbulence causes carbon dioxide (CO2) to escape from the water, changing its chemistry. The reduction in CO2 triggers the precipitation of calcium carbonate (CaCO3) crystals.
These tiny calcite crystals need a surface to attach to, which is provided by a community of microscopic organisms. A sticky biofilm created by cyanobacteria and diatoms, along with aquatic mosses, traps the precipitating calcite. The mosses, particularly species like Palustriella commutata and Ptychostomum pseudotriquetrum, are essential. As new moss grows on top of the old, the lower layers become encrusted and petrified, forming new layers of the travertine dam.
A 7,000-year-old process
Radiocarbon dating shows the active travertine barriers began forming around 6,000 to 7,000 years ago, after the end of the last Ice Age created a warmer, wetter climate suitable for this deposition. Geologists have found paleobarriers—the remnants of ancient dams—at higher elevations that date back 90,000 to 300,000 years, indicating this process has occurred here during previous interglacial periods.
The 16 lakes are divided into the Upper Lakes and Lower Lakes. The twelve Upper Lakes were formed on a substrate of dolomite rock, while the four Lower Lakes carved their path through limestone cliffs. The entire system descends over 130 meters from the first lake, Prošćansko Jezero (636 m), to the last, Novakovića Brod (503 m).
The growth of the dams is not uniform. Some barriers grow faster than others, altering water levels and redirecting flows. A submerged barrier in Kozjak Lake shows this process. Around 400 years ago, Kozjak was two separate lakes divided by a 40-meter waterfall. The dam at the final outflow of Kozjak grew much faster than the one separating the two smaller lakes, causing the water level to rise and submerge the interior waterfall, uniting them into the single large lake seen today. This constant, differential growth ensures that the appearance of the lakes and waterfalls is always changing.