Why the water is so clear, why each lake seems to have a different jewel-like color?
The official park map explains that over thousands of years, super-pure water pouring into Plitvice from the mountains eroded the surrounding limestone rocks, and moss and algae acted as catalysts to turn the dissolved rock (calcium carbonate) into the porous stone known as travertine, which then was deposited in the water. Nature is still "manufacturing" travertine at Plitvice and the dissolve-and-deposit process is responsible for creating the park's waterfalls and for the lakes' vivid blue-green color. (As the travertine sinks to the lake floor, it coats vegetation and the lake's floor in white mineral "dust," which creates a bottom that reflects sunlight and sky. Thus the brilliant colors.)
As the travertine accumulates on the bottom, it also builds to the point that it creates barriers between the lakes. These barriers eventually get tall enough to create the waterfalls that are the essence of Plitvice's constantly shifting landscape.