Rivers are the blue arteries of the Earth. Their flows deliver sediment and nutrients to floodplains, deltas and coastal zones, some of the most biologically productive ecosystems on the planet.  They connect and sustain the web of life.

So it might be surprising that globally we don’t systematically monitor their health.  Imagine damming and diverting the arteries in our bodies without taking care to monitor the consequences.  Our health would turn precarious, to say the least.

That rivers are suffering is not in doubt.

Today, 50,000 large dams block major rivers around the world, up from 5,000 in 1950.   That means on average we’ve been building two large dams a day for half a century – a massive change in the hydrological environment in what is geologically a twinkling of an eye.

Dams and their reservoirs now intercept about 35% of river flows as they head toward the sea, up from 5% in 1950.  They trap more than 100 billion tons of nutrient-rich sediment that would otherwise have replenished deltas and coastal zones.

A number of major rivers are so heavily dammed and diverted that they no longer reach the sea for all or part of the year, including the Colorado, the Indus and the Nile.

For aquatic life, there has been little time to adapt to these changes. Combined with pollution, habitat degradation has pushed many species to the brink of extinction, or beyond.  In North America, 40% of freshwater fish species are now to some degree at risk of extinction, and over the last two decades the number of listed species nearly doubled, to 700.

In light of these global trends, International Rivers, a non-profit river protection organization based in Berkeley, California, and with offices on three other continents, has developed an informative and engaging online tool called the State of the World’s Rivers that provides a health check-up for the world’s 50 largest river basins.

The tool groups indicators of river health into three categories – river fragmentation, biodiversity, and water quality.  Among the basins ranking highest in fragmentation and lowest in water quality – and therefore most in need of remediation – are the Danube, Indus, Mississippi, Godavari, Tigris-Euphrates, Volta and Yellow.

By contrast, river basins most important to preserve because of their high levels of biodiversity and water quality, and relatively low degree of fragmentation, include the Amazon, Congo, Mekong, Orinoco, Paraná, Tocantins, Yangtze and Zambezi.

The motivation for the project, explains Jason Rainey, executive director of International Rivers, was the growing “dam rush” in important river basins, especially in the Global South, where plans to double hydroelectric generation by 2050 could require 9,000 additional large dams.

The results of this first-round assessment lead the group to call for the convening of an international expert panel to undertake a more complete assessment of river health and to develop metrics that flag when rivers are in critical danger, much like thresholds for cholesterol or blood pressure.

Without question, dams have produced benefits.  They generate hydroelectricity, control floods, supply water for drinking and irrigation, and offer recreational opportunities.

But any serious analysis of dams must focus on the full range of benefits and costs – including not only an accurate rendering of economic costs but also of the ecological impacts and social harms done by the dam’s construction and operation.

In this regard, many dams fall very short of the mark.

Some 40-80 million people, mostly poor, have been displaced by large dams, often without compensation or adequate resettlement, and rarely reaping any of the direct benefits of the dam that displaced them, such as electricity.

In addition, a 2010 study I co-authored, published in the journal Water Alternatives, estimated that nearly half a billion people living downstream of dams have likely been negatively impacted as well – for example, by the loss of fisheries or riverside farming and grazing lands.  Rarely has there been any accounting of these social and economic costs, much less compensation for them.

Last month, Jacques Leslie, author of Deep Water: The Epic Struggle over Dams, Displaced People, and the Environment, wrote in a New York Times opinion piece that big dams are “Industrial Age artifacts that rarely deliver what they promise.”

He reported on a new study by Oxford University researchers who analyzed 245 large dams built between 1934 and 2007 and found long construction delays and huge cost-over-runs.  On average, actual dam expenses ran nearly double pre-construction cost estimates.

Dams are just one cause of rivers’ deteriorating health, but a crucially important one.

Spending a little time with the State of the World’s Rivers online platform quickly reveals how seriously rivers are in trouble – and how urgently the world needs to monitor and protect the blue arteries of the Earth.

Originally published at National Geographic Newswatch.