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Water trapped behind dams also loses most of its sediment load, so suspended sediment is no longer available to nourish downstream deltas, floodplains, and estuaries. The Colorado River was so named because the word means “colored” in Spanish, in reference to the huge amounts of red sediment the river once transported. But since the closure of the Glen Canyon Dam this once-turgid river has flowed crystal-clear, nearly eliminating native fish species that were adapted to feeding in muddy waters without eyesight. (Only three out of eight native fish species that were abundant before 1963 are still common today—the rest are extinct or nearly so.) Worldwide, reservoirs trap about a third of the sediment that would otherwise flow down rivers, and more than 90 percent in heavily dammed systems like the Nile, the Colorado, the Rio Grande, and the Volga.21

Where river deltas are denied a constant influx of sediment, they lose the battle against the ocean and begin—slowly but irrevocably—to sink. This is bad news for the megacities built on deltaic plains like Shanghai on the Yangtze Delta, Calcutta and Dhaka on the Ganges–Brahmaputra, Lagos on the Niger, and Ho Chi Minh City on the Mekong Delta. As these cities approach or even drop below the level of the rising seas, the risk of flooding intensifies: 10 million people a year currently suffer from storm surges, mostly in Asian deltas. Direct hits by hurricanes can be catastrophic, as New Orleans discovered during the Hurricane Katrina disaster in 2005. In the Burmese Irrawaddy Delta, a storm surge is estimated to have killed upwards of 100,000 people during Cyclone Nargis in 2008. Deltas where rivers no longer reach the sea get no sediment deposition at all, like the rapidly disappearing and ecologically sensitive Colorado delta in Mexico.

Given all of these ecological impacts, and the natural upper limit set by the amount of fresh water flowing through the world’s rivers at any one time, humanity has little option but to recognize that water must be shared between our species and the rest of the biosphere if major and accelerating impacts on the Earth system are to be avoided over the long term. The question is where this level should be set, and the planetary boundaries expert group suggests a quantified boundary, expressed in terms of the human consumptive use of fresh water, of 4,000 cubic kilometers per year. Current human consumptive water use (meaning water used, evaporated, or incorporated into a crop, so not available for direct reuse) is estimated at around 2,600 km3.22 So on a planetary level, humanity is still within the confines of what can be considered sustainable, despite all the specific environmental impacts listed above.

As with the land use boundary, this is qualified good news. It means that there is still some room for expansion of human consumption before we run up against hardwired limits in the Earth system in some areas. While on climate, biodiversity loss, and nitrogen we are well over the levels set for sustainable long-term use, we can still tap some rivers to bring fresh water to those who currently lack it and to grow more food for a still expanding population. But where this water is taken from matters hugely: While greenhouse gases have the same impact wherever they are around the planet, water removed from an arid region like the Middle East will be much more damaging than the same amount captured from a river in well-watered northern Canada. This planetary boundary is therefore perhaps best thought of as an aggregate total of many smaller boundaries operating at the scale of single river systems in different continents and regions, rather than as a single global number in and of itself.

WET AND WILD

The most radical solution for restoring rivers to ecological health is simply to tear down unnecessary dams, revitalizing the natural hydrology and allowing migrating fish to return. Anywhere this can be done it should be, and much evidence suggests that a substantial fraction of currently existing dams could