This story is part of a special National Geographic News series on global water issues.
The Colorado River is one of the most used and contested waterways on Earth. It provides water for 30 million people, and has many dams and diversions along its 1,450 miles (2,333 kilometers).
Because it is so heavily tapped for agriculture, industry, and municipal uses along its course, the Colorado River rarely reaches its delta and the Gulf of California. About one-tenth of the river's former flow now makes it to Mexico, but most of that is used for farming and cities south of the border.
A growing coalition of advocates, including National Geographic, is working to restore some of the water in the Colorado, with hopes of regenerating the now-arid delta (previous image) and important ecosystems along the way.
More people have come to appreciate the vital role the river plays on both sides of the border. There is growing interest in removing some of the dams along its path, including the controversial Glen Canyon Dam near the Grand Canyon.
Peering from a decoy, a hunter lifts his head above the water of the Indus River. The Indus is the primary source of freshwater for most of Pakistan, a fast-growing nation of more than 170 million people.
Waters from the Indus are drawn for household and industrial use, and support about 90 percent of the agriculture in the arid country. The Indus is one of the great rivers of the world, but it is now so exploited that it no longer flows into the ocean at the Port of Karachi.
Instead, in the words of New York Times writer Steven Solomon, the Indus is "dribbling to a meager end . . . Its once-fertile delta of rice paddies and fisheries has shriveled up." The lower Indus had been a lush ecosystem, supporting artisanal fishers and providing habitat to diverse species, including the critically endangered Indus River dolphin.
Choked off from its water supply, Karachi is plagued by increasingly brazen water thieves and riots over scarcity. Many in the water-stressed delta blame wealthy landowners upstream for taking water out of the river. As National Geographic News recently reported, tensions have been running high with neighboring India, which is home to the glaciers that feed the river, and which is planning more large-scale diversions.
Solomon concludes that Pakistan's water future is "grim," with the population expected to pass 220 million within a decade, and flows of the Indus to fall further in the wake of global warming. He points out that the country currently only has the capacity to store enough water for about 30 days’ use.
Even so, as National Geographic News reported, there are signs that India and Pakistan are cooperating better over water than they have in the past. Population growth in the region has been slowing, and there is rising awareness about the importance of protecting the world's great rivers.
Amu Darya River
Many schoolchildren know the sad tale of the Aral Sea, once the world's fourth largest inland water body with a surface of 26,000 square miles (67,300 square kilometers). The sea was once ringed with prosperous towns and supported a lucrative muskrat pelt industry and thriving fishery, providing 40,000 jobs and supplying the Soviet Union with a sixth of its fish catch.
The Aral Sea was originally fed by two of Central Asia's greatest rivers, the Amu Darya in the south and the Syr Darya in the north. The former is the longest river in the region, snaking through 1,500 miles (2,414 kilometers) of steppe.
But in the 1960s, the Soviets decided to make the steppes bloom. So they built an enormous irrigation network, including 20,000 miles of canals, 45 dams, and more than 80 reservoirs, all to irrigate sprawling fields of cotton and wheat in Kazakhstan and Uzbekistan. The system was leaky and inefficient, however, and after several decades, the Amu Darya had lost so much of its flow that it no longer reached the Aral Sea. Today, it ends about 70 miles (110 kilometers) away.
Pictured is the Amu Darya a little ways upstream from where it dries out.
Although the Syr Darya fared somewhat better than its sister river, the Amu Darya, it was also heavily tapped and polluted. The Syr Darya starts in the Tian Shan Mountains of Kyrgyzstan and Uzbekistan and flows 1,374 miles (2,212 kilometers) toward what's left of the Aral Sea. (Pictured is a reach near Tashkent.)
In the 18th century, a system of canals was built on the river. These structures were greatly expanded by Soviet engineers during the 20th century, largely to grow vast quantities of cotton. Virtually the entire flow was diverted, leaving only a trickle into the inland sea.
The deputy director of Kazakhstan's agency for applied ecology, Malik Burlibaev, recently warned that "the Syr Darya is so polluted that water from it should not be used for drinking or for irrigation."
In the past few years, the World Bank has funded a dam and restoration project with the goal of improving the health of the Syr Darya and increasing the flow into what's left of the North Aral Sea.
Rio Grande River
One of the largest rivers in North America, the 1,885-mile (3,033-kilometer) Rio Grande runs from southwestern Colorado to the Gulf of Mexico. It defines much of the border between Texas and Mexico. But the once grande river is looking more poco these days, thanks to heavy use on both sides of the border.
Less than a fifth of the Rio Grande's historical flow now reaches the Gulf. For a few years in the early 2000s, the river failed to reach the coast entirely. All that separated the United States from Mexico was a beach of dirty sand and an orange nylon fence.
Here, the river defines the international border across the Adams Ranch near Big Bend National Park.
Algae colors the confluence of the Rio Grande and Arroyo San Carlos.
The population in the Lower Rio Grande Valley is exploding in both the United States and Mexico, driven by NAFTA-era factories and agricultural productivity. But by the time it reaches Matamoros, the river's level is so low that it often falls below the Mexican city's intake pipes. Farmers in Texas say they lose $400 million annually due to lack of irrigation water.
The region's wetlands, once critical stopover points for migrating birds, are getting choked off. All these problems are made worse by the decades-long drought gripping the region.
The Yellow River is the second longest in China, after the Yangtze, and the sixth longest in the world, with a course of 3,395 miles (5,464 kilometers). The Yellow River was the cradle of the earliest known Chinese civilization, and it has a long and complex history in the region. Numerous floods over the centuries resulted in catastrophic loss of life, including a flood in 1931 that killed one to four million people.
Since 1972, the Yellow River has frequently run dry before reaching the sea, thanks to extensive diversion, largely for agriculture. In 1997, the lower Yellow River did not flow for a whopping 230 days. Such a dramatic decrease in water has choked off the ecologically rich delta, which is also eroding due to loss of silt.
In recent years, the Chinese government has taken steps to restore some of the water's flow, denying some farmers use along the way.
The Yellow River carries an unusually high amount of silt, so much that it deposits a layer through much of its course. This raises the level of the riverbed, in some cases above surrounding land. Eventually, the natural levees that develop fail, leading to massive flooding. The river also has tended to shift courses about once every hundred years.
There are several dams on the river, but their life expectancy is lessened due to the heavy load of silt.
Pictured, officials discharge as much as 3,500 cubic meters per second of water to try to flush out silt at the Xiaolangdi Reservoir. More than 300 million tons of sand from the lower Yellow River were flushed into the sea during four previous sand-washing operations.
The Teesta River flows 196 miles (315 kilometers) through the Indian state of Sikkim and into the Brahmaputra River in Bangladesh. It starts in the Himalaya, where it is fed by snowmelt, and then carves its way through temperate and tropical valleys.
The Teesta is often called the lifeline of Sikkim, but in recent years it has been so tapped for irrigation and other uses that it has largely dried up. Fishermen are no longer able to make a living along its banks, and thousands of farmers have lost their water supply.
Even so, India is going ahead with plans to build a new series of dams along the Teesta, in a bid to generate electricity. Geologists warn that the weight of sediments that pile up as a result could trigger earthquakes in the seismically active area.
Some experts have warned that the troubles in Australia's Murray River Basin may be a harbinger of what other water-stressed regions can expect in a warming world, with rising human population. The Murray is Australia's longest and arguably most important river, stretching for 1,476 miles (2,375 kilometers) from the Australian Alps, across the inland plains to the Indian Ocean near Adelaide.
As a result of irrigation, the Murray Valley is Australia's most productive agricultural zone, and is widely known as the nation's food bowl. However, withdrawals have resulted in rising salinity, which threatens that agricultural productivity. The river is also the source of 40 percent of Adelaide's drinking water and most of the water for many smaller towns along its length.
Disruptions and diversions have reduced the flow so much that the mouth of the river closed due to silt formation at the beginning of the 21st century. Only dredging is able to keep the final channel open, both to the sea and the lagoon of nearby Coorong National Park.
The mouth of the Murray River, where a dredge keeps the pathway open.
The Murray faces other serious environmental threats, including polluted runoff, especially from farms in four Australian states, and introduction of invasive species, especially the European carp.
Similar problems affect the Darling River, which flows into the Murray at Wentworth. The Darling is known as a main waterway of the outback, but some years it is so tapped and affected by drought that it hardly flows at all.