Worlds of Rivers

Few lands are untouched by the power of a stream. Even the driest regions can hold traces of ephemeral river runs. Rivers create their own channels, gathering rain or snow and ice melt that runs off the land and bearing it downhill by force of gravity to the sea. The journey makes rivers the unsurpassed carvers of Earth, cutting canyons and valleys and depositing sediments as fertile soils. By nature, they wind—the most efficient way for water to travel. Rivers and lakes store less than half a percent of Earths fresh water, but they are the lifelines of human history—where people settled, farmed, traded, built cities, explored. A new chapter in river mapping reveals the true intricacies of river flow as headwaters feed consecutively larger tributaries that surrender their waters to the mainstream. Most major rivers are now yoked with dams and reservoirs, but on this map, rivers run free.

Ten Longest Rivers
1. Nile (Africa) 4,400 miles
2. Amazon (South America) 4,150 miles
3. Yangtze (Asia) 3,880 miles
4. Mississippi-Missouri (North America) 3,780 miles
5. Yenisey-Angara (Asia) 3,610 miles
6. Yellow (Asia) 3,590 miles
7. Ob-Irtysh (Asia) 3,430 miles
8. Amur (Asia) 3,420 miles 

Interesting facts:

The most international river system, the Danube, and its tributaries are used by more than 80 million people in 20 European countries. Winding east from Germany to the Black Sea, it is the continent’s second longest river, after Russia’s Volga.

The world’s longest river, the Nile, and Africa’s equatorial colossus, the Congo, both rise in the highlands where Africa's greatest lakes fill parts of the Great Rift Valley. The Congo, its volume second to that of the Amazon, is the only major river that crosses the Equator twice.

Almost 70 percent of Earth's fresh water is frozen in ice sheets, glaciers, permanent snow cover, and permafrost. Antarctica holds about 90 percent of that water, with most of the rest locked up on Greenland.

Sediment carried from the Himalaya and the Tibetan Plateau creates some of the world’s largest river deltas, including the Indus, the Mekong, and the great delta where the Ganges meets the Brahmaputra River system. Worldwide, deltas are home to an estimated 500 million people.

China's Yangtze River powers the Three Gorges Dam, unequaled in hydroelectric generating capacity. Another monumental project is now underway to divert water from the Yangtze to China’s dry, populous north. There, the Yellow River, drawn down for irrigation and industry, often fails to reach the sea.

The Murray-Darling is Australia’s only major river system, rising in the mountain ranges of the southeast. Heavily tapped for irrigation, its flow has declined during a decade of drought. The continent’s intermittent rivers and lakes can fill if water flows south after heavy seasonal rains in the tropical north.

Most of the planet’s liquid freshwater is groundwater, precipitation that seeps down to fill the spaces in layers of sand, gravel, and permeable rock called aquifers. Groundwater exists almost everywhere, at varying depths. Since the mid-20th century, its extraction for human use has accelerated, often at unsustainable rates. How readily groundwater recharges depends on precipitation, geology, and topography. Groundwater can emerge as a spring, the start of many rivers and for some a major contributor. Up to 40 percent of the volume of the Mississippi River is estimated to come from groundwater.

Source: National Geographic

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