Within the boundaries of this heavily forested national park, sixteen crystalline lakes tumble into each other via a series of waterfalls and cascades. The mineral-rich waters carve through the rock, deposing tufa in continually changing formations. Clouds of butterflies drift above the eighteen km of wooden footbridges and pathways which snake around the edges and under and across the rumbling water. The lakes are situated on the eponymous Plitvice plateau, between the mountains of Lička Plješevica, Mala Kapela and Medveđak. The sixteen lakes are separated into an upper and lower cluster formed by runoff from the mountains, descending from an altitude of 636 m to 503 m over a distance of some eight km, aligned in a south-north direction. The lakes collectively cover an area of about two km², with the water exiting from the lowest lake to form the Korana River. The Plitvice Lakes lie in a basin of Karstic rock, mainly dolomite and limestone, which has given rise to their most distinctive feature. The lakes are separated by natural dams of travertine, which is deposited by the action of moss, algae and bacteria. The encrusted plants and bacteria accumulate on top of each other, forming travertine barriers which grow at the rate of about 1 cm per year. The lakes are renowned for their distinctive colors, ranging from azure to green, grey or blue. The colors change constantly depending on the quantity of minerals or organisms in the water and the angle of sunlight. The lakes are divided into the 12 Upper Lakes and the four Lower Lakes.