Coulee lakes are also of volcanic origin but they are formed by the flow of lava across the valley of some river so that it blocks the river flow and gives rise to lakes. Lake Tana of Absyssinia is a characteristic example. Several lakes of this type are found in Iceland. Such lakes are usually rectangular in outline but may be irregular also. Sink lakes are characteristic of limestone regions, where the water descending through the shallow holes produces large underground caves. In case of roof of such a subterranean cavern collapses, a basin is formed. Such a basin resembles a sink and has also been formed by the sinking of the land. The lakes formed in these depressions are known as ‘Sink Lakes’. These are usually small in size.
The Amazon varies in width from 4 to 6 miles (6 to 10 kilometers); its mouth is more than 150 miles (240 kilometers) wide. The largest oceangoing steamers can ascend the river 1,000 miles to Manaus, a Brazilian inland port.
Plants and Animals
The remarkably rich and diverse Amazon Basin plant and animal life is a resource of world importance. Of all the species of plants in the world, almost three fourths, many of which are still unidentified, live in the Amazon Basin. The Amazon has often been described as a vast sea of fresh water that supports about 1,500 to 2,000 species of fish, including catfish, electric eels, and piranhas. The basin also has an immense variety of insect, bird, reptile, and mammal life.
Mammals include the capybara, a rodent weighing up to 110 pounds (50 kilograms) whose flesh is eaten; the tapir, an edible kind of pig; the nutria, a tropical otter whose pelt is traded; the great anteater; and many kinds of monkeys. Markets along the river sell a variety of fish, including the pirarucu, which weighs up to 325 pounds (150 kilograms), and the giant catfish. Silver carp, neon tetras, and the flesh-eating piranhas are shipped to tropical fish stores throughout the world. The electric eel is a dangerous fish capable of discharging up to 500 volts.
The wide range of vividly colored Amazonian birds includes hummingbirds, toucans, and parrots. Among the reptiles are the anaconda, a huge snake that crushes its victims; the poisonous coral snake; and alligators. Giant butterflies are among the most spectacular of the insects.
Biosphere reserves are ‘areas of terrestrial and coastal/ marine ecosystems or a combination thereof, which are internationally recognised within the framework of UNESCO’s Programme on Man and the Biosphere (MAB)’.
Reserves are nominated by national government; each reserve must meet a minimal set of criteria and adhere to a minimal set of conditions before being admitted to a before being admitted to a network.
Each biosphere reserve is intended to fulfil three complementary functions: a conservation function, to preserve genetic resources, species, ecosystems and landscapes; a development function, to foster sustainable economic and human development, and a logistic support function, to support demonstration projects, environmental education and training, and research and monitoring related to local, national and global issues of conservation and sustainable development.
A place where water naturally flows to the surface is called a Spring. Springs differ between themselves by the conditions of natural underground water discharge depending on such factors as the nature of the water- bearing rocks (porous or fractured), the exposure of the river or gulley slope, the mode of occurrence of the rocks etc. Sometimes, springs emerge at the sea floor and are called ‘Submarine Springs’.