The spectacular Plitvice Lakes are actually part of one large river flowing between the Mala Kapela and Licka Plješivica mountains in central Croatia.
The river has divided into this series of interconnected lakes and waterfalls because of a geological phenomenon known as a karst landscape, where rock, water and organisms all work together to create new features.
The Plitvice river basin is made of limestone and dolomite, and as the water passes through it dissolves these rocks and becomes saturated with calcium carbonate. This chemical compound then sticks to the mucus secreted by the microscopic bacteria and algae that grow on moss plants in the water. The plants gradually become encrusted with the calcium carbonate and it slowly builds up at a rate of about one centimetre (0.4 inches) per year to form barriers of travertine rock. Some of these barriers, which have been growing since the Upper Triassic period, are around 4,000 meters (13,123 feet) thick and act as natural dams that split the river into lakes. As more water travels down from the mountains, it flows over these barriers to create waterfalls that cascade down the river basin.
Just as quickly as the flowing water erodes the travertine, more is formed when the calcium carbonate-saturated water pools at the base of the waterfalls. This means that the Plitvice Lakes are constantly changing size and shape as old waterfalls run dry and new ones form.
This clever geology is also responsible for giving the Plitvice Lakes their distinctive blue-green colour. When the white calcium carbonate coats the bottom of the lakes it reflects sunlight and the sky to create vivid colours that change depending on how the Sun’s rays hit the water and how many organisms and minerals are present.
How does a waterfall form?
Usually, waterfalls form when a river flows over areas of soft and hard rock. The flowing water erodes the soft rock more quickly than the hard rock, undercutting it to leave an overhang. This forms a basic waterfall, and as the water flows over this ledge, it often takes some rocks with it. These rocks crash into the riverbed below, so more erosion occurs to form a plunge pool. The soft rock behind the waterfall is also eroded as water splashes at the bottom, cutting into the rock to form a cave-like structure called a rock shelter. Eventually the water erodes the overhang of hard rock too, causing the waterfall to recede upstream. This process is slightly different to that which occurs at the Plitvice Lakes, as instead of carving existing rock into an overhang, the mineral-rich water there helps to create new ledges that the water can flow over.
Discover more amazing technology in the latest issue of How It Works. It’s available from all good retailers, or you can order it online from the ImagineShop. If you have a tablet or smartphone, you can also download the digital version onto your iOS or Android device. To make sure you never miss an issue of How It Works magazine, make sure you subscribe today!