River deltas—like those at the mouth of the Mississippi, Nile, or Ganges—barely rise above sea level. Among the regions most unchanged by climate change, they barely rise to the level of public attention. That’s unfortunate because deltas—which form where large rivers deposit sediment as they flow into the ocean—are home to half a billion people. They also support some of the planet’s most productive agricultural regions and fish harvests.
Our warming planet projects 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 you’ll see that it is brown. A river is brown because it is more than just a flow of water, it is also a flow of sediment. These sediments, including silt and sand, are the products of erosion across a river’s 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.