Gần như tất cả "nước ngọt" chứa muối của một số loại, hòa tan từ đá và đất. Khi nước bốc hơi từ hồ, nó để lại các muối phía sau, và trong một khí hậu sa mạc nóng này có thể tạo ra một hồ muối. | o SALT LAKE Nearly all fresh water contains salts of some kind dissolved from rocks and soils. As water evaporates from lakes it leaves these salts behind and in a hot desert climate this can create a salt lake. The waters of the Great Salt Lake in Utah are five times as salty as the sea and the margins of the lake seen here are encrusted with glittering white salt crystals. o SODA LAKE Typical salt lakes are rich in sodium chloride or table salt. But some lakes contain other salts. Many lakes in Africa s Rift Valley such as Lake Nakuru contain strong solutions of sodium carbonate or soda. Despite this the lake water supports a dense population of specialized life including microscopic algae and shrimplike copepods which are eaten by vast flocks of flamingos. o GLACIAL LAKE Most of the world s lakes were formed by ice-age glaciers. The moving ice scooped hollows in the rock or dumped thick moraines of rocky debris in valleys that now act as natural dams holding back the lake water. Similar lakes are being formed today by active glaciers like this one in southern Norway. Meltwater flowing from the glacier in the background is rich in mineral sediment which gives the lake its greenish blue color. o CRATER LAKE The craters of extinct or dormant volcanoes often contain near-circular crater lakes. They fill with pure rainwater but if there is any volcanic activity the water may become acidified by gases such as sulfur dioxide and carbon dioxide. The water of this crater lake in eastern Siberia is unusually acidic enabling it to dissolve the minerals that have turned it a milky blue. 78 CAVES AND UNDERGROUND RIVERS A POTHOLES The narrow passages that link bigger caves are known in some limestone regions as potholes. Their walls are often visibly scoured and polished by the torrents of water that flow through them after heavy rain and many are full of water all the time. This does not stop determined cavers who use specially modified diving equipment to pass through .
