We have truly lost our way when our rivers can no longer find the oceans

According to a new United Nations study, human interference with the world's great waterways has altered ecosystems and is driving species to extinction

The Three Gorges Dam in China's Hubei province is the world's largest. AFP

Major river systems are the lifelines of large sections of the world’s population. They not only supply the most essential of all natural resources – water – but also sustain biodiversity, which in turn supports human beings.

However, the old saying, “Follow the river and it will eventually lead you to a sea”, is no longer wholly true. Owing to excessive damming and drastic overuse of water resources, an increasing number of rivers are drying up before they reach the sea.

A new United Nations study offers grim conclusions: human actions are irremediably altering rivers and other ecosystems and driving increasing numbers of plant and animal species to extinction. According to the study's summary of findings, released last week: "Nature across the globe has now been significantly altered."

Water sustains life and livelihoods and enables economic development. If the world is to avert a thirsty future and contain the risks of greater intrastate and interstate water conflict, it must protect freshwater ecosystems, which harbour the greatest concentration of species.

Yet, according to another study published in this month's Nature journal, humans have modified the flows of most long rivers, other than those found in the remote regions of the Amazon and Congo basins and the Arctic. Consequently, only about one third of the world's 246 long rivers can still be described as free-flowing, meaning that they remain clear of dams and other man-made diversions.

Instead of flowing freely, rivers are becoming increasingly fragmented by dams and other hydro-engineering structures. Such fragmentation is affecting the flow of nutrient-rich sediment from the mountains where rivers originate, riverside vegetation, the migration of fish and quality of water.

For example, the Colorado River, which is broken up by more than 100 dams and thousands of kilometres of diversion canals, has not reached the sea since 1998. The river, which originates in the Rocky Mountains and is the lifeblood for the south-western United States, used to empty into the Sea of Cortez in Mexico. Now, because of the upstream diversion of 9.3 billion cubic metres of water annually, the Colorado's flow into its delta has been reduced to a trickle.

Others that run dry before reaching the sea include the Yellow River, the cradle of the Chinese civilisation; the Amu Darya and the Syr Darya – the two lifelines of Central Asia; the Euphrates and the Tigris in the Middle East; and the Rio Grande, which marks the entire Texas-Mexico border before heading to the Gulf of Mexico. The over-utilised Murray in Australia and the Indus in Pakistan are at risk of meeting with the same fate.

Shared water resources are often siphoned off by upstream powers with little consideration for the interests of downstream states. For example, Mexico has long complained that it is not getting its share of the Colorado River’s waters under the terms of a 1944 water-sharing treaty with the US.

China's hyperactive dam building illustrates the high costs of river fragmentation

More fundamentally, altered flow characteristics of rivers are among the most serious problems for sustainable development, because they seriously affect the ecosystem services on which both humans and wildlife depend. Free-flowing rivers, while supporting a wealth of biodiversity, allow billions of fish – the main source of protein for the poor – to swim through their waters and breed.

Free-flowing rivers also deliver nutrient-rich silt crucial to agriculture, fisheries and marine life. Such high-quality sediment helps to naturally refertilise overworked soils in the plains, to sustain freshwater species and, after rivers empty into seas or oceans, to underpin the aquatic food chain supporting marine life.

China’s hyperactive dam building illustrates the high costs of river fragmentation. No country in history has built more dams than China. Today, it has more large dams than the rest of the world combined.

China’s chain of dams and reservoirs on each of its long rivers impedes the downstream flow of sediment, thereby denying essential nutrients to agricultural land and aquatic species. For example, by disrupting silt flows in the Yangtze River, the Three Gorges Dam – the world’s largest – has caused a problematic build-up of sediment in its own massive reservoir.

Likewise, China's eight giant dams on the Mekong, just before the river enters south-east Asia, is affecting the quality and quantity of flows in the delta in Vietnam. An undeterred China, however, is building or planning another 20 dams on the Mekong.

How the drying up of rivers impacts seas and oceans is apparent from the Aral Sea, which has shrunk 74 per cent in area and 90 per cent in volume, with its salinity growing nine-fold. This is because the Aral Sea's principal water sources, the Amu Darya and Syr Darya, are so over-exploited for irrigation that they dry up before reaching what was once the world's fourth-largest inland lake.

Then there is the matter of pollution, which, along with the disruption of natural river flows, has adversely affected traditional agriculture and grazing, devastated fisheries and marginalised rural communities.

Meanwhile, the continued shrinkage and degradation of freshwater habitats – including rivers, lakes, wetlands and ponds – is accelerating biodiversity loss extending to the seas. Aquatic ecosystems have lost 50 per cent of their biodiversity since the mid-1970s alone.

Urgent action is needed to save the world’s rivers. It must be taken on several fronts, including improving practices in agriculture, which accounts for the bulk of the world’s freshwater withdrawals. Without embracing integrated water resource management and other sustainable practices, the world risks a parched future.

Brahma Chellaney is the author of nine books, including Water, Peace, and War.