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world's major deltas threatened by climate change - and also by how we develop hydropower

world's major deltas threatened by climate change - and also by how we develop hydropower

River deltaslike those at the mouth of the Mississippi, Nile, or Gangesbarely rise above sea level. Among the regions most imperiled by climate change, they barely rise to the level of public attention.

Thats unfortunate, because deltaswhich form where large rivers deposit sediment as they flow into the oceanare home to half a billion people. They also support some of the planets most productive agricultural regions and fish harvests.

Our warming planet poses an existential threat to deltas, a reality made clear in the Special Report on the Ocean and Cryosphere in a Changing Climate, released this week from the Intergovernmental Panel on Climate Change (IPCC).

The IPCC report states with high confidence that deltas will face high to very high risks in the future from rising sea levels, even under scenarios where the world rapidly reduces emissions of greenhouse gases and minimizes the rise in global temperatures and the subsequent melting of ice sheets and glaciers.

But the report also identifies other threats to deltas, including the loss of the sediment needed to replenish them and keep them above the rising seas. And while climate change requires a global solution, sediment loss has solutions that are far more local and arguably more tractable in the short term. Ironically, a key contributor to this more local threat is often billed as one of the solutions for climate change: hydropower dams.

Use Google Earth to look at nearly any large river on Earth and youll notice that it is brown (see photo below). A river is brown because it is more than just a flow of water moving downstream, it is also a flow of sediment. These sediments, including silt and sand, are the products of erosion across a rivers basin (all the land that eventually drains into the river). Imagine sand eroding from a mountainside in Montana or silt sliding off a farm field in Missouri: all of that sediment will eventually end up in the mud-colored Mississippi as it glides past New Orleans and out to the ocean.

When a river reaches the ocean, it loses its ability to continue carrying its sediment load and most of that silt and sand is deposited. Over long periods of time, this process results in the creation of new land, often in the general shape of a triangle (hence the name delta after the Greek letter; see image of Nile Delta, below at right).

On the left, the delta of the Ganges Riverwhere the main river breaks up into multiple smaller... [+] channelsillustrates how rivers carry sediment and deposit it where they flow into the sea, building deltas. On the right, the Nile River flows north (toward top of photo) into the Mediterranean Sea forming a classic triangular shaped delta, clearly demarcated by the dense lights. Nearly half of Egypts population lives in the Nile Delta; upstream dams now capture 98% of the sediment supply that would otherwise flow toward the Delta to maintain it.

Actually, today its not quite accurate to say that most of that Montana sand and Missouri silt is deposited in the Mississippi River Delta. A sizable fractionnearly halfof that sediment gets captured along the way in the reservoirs on the Missouri River and other major tributaries of the Mississippi. A large dam backs up a reservoir which greatly slows or halts a rivers flow, causing its sediment to drop out. These reservoir deltas expand at the expense of the river delta downstream (see photos below).

On the left, the Srepok River, a tributary of the Mekong River, flows into a reservoir created by... [+] the Lower Sesan II Dam (identified on the left of the photo). The Srepok was once one of the largest contributors of sediment to the Mekong Delta, but now that sediment is captured in the reservoir (note the muddy plume as the Srepok enters the reservoir on the right side of the photo). As the sediment supply to the Mekong Delta diminishes, including due to extensive sand mining, erosion rates have increased (photo on right).

The loss of sediment supply robs a delta of the raw material it needs to keep pace with a range of forces intent on tearing it down, including rising sea levels and waves and storms that erode land (note that other processes also reduce sediment supply to deltas, including unsustainable sand mining, but here we focus on capture by reservoirs). With diminished deliveries of sediment, deltas begin to shrink and sink (see photo above).

While the Mississippi River Delta has lost about half of its sediment supply due to upstream reservoirs, many river deltas have lost nearly all their sediment. For example, the deltas of both the Colorado River and the Nile River have lost more than 98% of their sediment supply. The Nile Delta is home to more than 40 million people (nearly half of Egypts population) and the countrys most important farmland (the image of night lights above illustrates dramatically the population density of the Nile Delta).

With 50,000 major dams worldwide, people now have a considerable impact on the global supply of sediment for deltas. Reservoirs trap approximately one quarter of the global annual flux of sedimentsilt and sand that would otherwise reach deltas and the ocean. And the problem is only going to worsen: thousands more dams, mostly for hydropower, are under development or proposed for many of the worlds rivers that still supply much or most of their original sediment loads to their deltas.

More than 500 million people live on deltas worldwide. The Mekong Delta provides a clear illustration of whats at stake for many of those peopleand some of the ways we may still be able to help them.

Historically, the Mekong River carried the tenth largest natural sediment load in the world, delivering 160 megatons (Mt) per year downstream to the delta, which is an economic powerhouse for the region. The Mekong Delta supports a population of 20 million people and approximately a quarter of Vietnams gross domestic product (GDP). Its cropland produces more than half of Vietnams staple crops and nearly 90% of its rice exports a significant amount given the country is one of the worlds leading rice exporters.

Hydropower development on the mainstem Mekong, particularly in China, and on many of its tributaries has already reduced delivery of sediment to the delta by approximately 70% (from 160 to 50 Mt/year). This loss of sediment (from capture behind dams, but also from unsustainable sand mining), combined with a range of other impacts, is contributing to a rapid increase in rates of erosion in the Mekong Delta.

Dozens more hydropower dams are under construction or planned for the Mekong and its major tributaries. Full development of those dams will further reduce sediment supply to less than 10% of the natural rate. That loss, along with rising sea levels and continued groundwater extraction, would result in most of the delta disappearingunder the ocean by the end of the century.

But hydropower dams do not need to have such a dramatic impact on deltas. Rafael Schmitt, a researcher at Stanford University, has found that more strategic siting of damsavoiding those locations where dams will have the biggest impacts on sedimentcould greatly minimize the loss of sediment to deltas. For example, Schmitt and colleaguesstudied hydropower development in a portion of the Mekongbasin that is the most important remaining contributor of sediment. They calculated that current, unplanned hydropower development has cut off nearly 90% of the sediment supply. But they also found that a more strategic approach could have achieved the same energy level while disrupting only 20% of the sediment supply (see figure below). Schmitt will soon publish similar research focused on the entire Mekong basin and reaching the same conclusion: a more strategic approach to developing hydropower could have maintained considerably greater sediment supply for the Delta for the same level of generation capacity, compared to the non-strategic set of dams that we have today.

For the 3S Basin (the largest remaining contributor of sediment to the Mekong basin), a strategic... [+] planning approach could have produced the same generation as the actual development trajectory, but maintained eight times as much sand exported from the basin to the downstream delta.

Further, the renewable revolutionthe dramatically dropping costs for solar and wind generation and storagehas increased the options for countries to meet growing electricity demand. Countries can now develop low cost and low carbon power systems without building hydropower dams that have significant conflicts with communities, rivers and deltas.

The IPCCs report makes it clear that climate change poses a significant risk to deltas, and the hundreds of millions of people who live on them, due to rising seas and extreme storms. But the report also makes it clear that deltas confront a range of other threats, including the loss of sediment supply coming from river basins. In some important deltas, like the Mekong, this threat is greater and more urgent than sea level rise.

Minimizing that threat is well within our reach and is fundamental to efforts to build more resilient deltas. If we ensure enough sediment continues to reach deltas, we will have time to solve the other diverse challenges they face. Addressing the sediment challenge will go a long way toward safeguarding these vitally important regions from the major threats of climate change. But if we do not solve this challenge, all our other efforts to save deltasincluding the construction of levees and floodwallswill amount to band-aids trying to hold back the sea.

As global lead scientist for freshwater, I work across WWFs network and with external partners to integrate scientific research into strategies for river conservation and sustainable energy development. My recent research has focused on the economic, financial and environmental dimensions of hydropower development and operation. My scientific and policy research has been published in journals such as Science, BioScience and Ecological Applications and am the lead author of the book Floodplains: processes and management for ecosystem services, published in 2017. I hold a PhD in ecosystem science from the University of California, Berkeley and a B.S. in biology from Duke University.

As global lead scientist for freshwater, I work across WWFs network and with external partners to integrate scientific research into strategies for river conservation and sustainable energy development. My recent research has focused on the economic, financial and environmental dimensions of hydropower development and operation. My scientific and policy research has been published in journals such as Science, BioScience and Ecological Applications and am the lead author of the book Floodplains: processes and management for ecosystem services, published in 2017. I hold a PhD in ecosystem science from the University of California, Berkeley and a B.S. in biology from Duke University.

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