Erica Gies writes:

Gazing out from the eighth floor of a hotel in Georgetown, Guyana, the broad expanse of the Atlantic Ocean was a muddy brown. Only a thin rim of blue on the horizon showed the ocean’s true color; the rest swirled with sediment emerging from the mouth of the Essequibo River.

In a rhythm that’s pulsed through epochs, a river’s plume carries sediment and nutrients from the continental interior into the ocean, a major exchange of resources from land to sea. More than 6,000 rivers worldwide surge freshwater into oceans, delivering nutrients, including nitrogen and phosphorus, that feed phytoplankton, generating a bloom of life that in turn feeds progressively larger creatures. They may even influence ocean currents in ways researchers are just starting to understand. But today, in rivers around the world, humans are altering this critical phenomenon.

In many places, the great culprit is the dam: a wall of concrete and stone bisecting a river, diverting its energy and water to human use. There are 58,000 “big dams”—50 feet high or taller—around the globe, with another 3,700 more planned, mostly in lower-income countries in Asia and South America.

Many of the harms caused by dams are well-documented. They block fish passage and starve subsistence fishers; radically alter natural river regimes and aquatic creatures’ lifecycles; and flood forests, wetlands, villages, and historical sites. (They’re also less climate-friendly and reliable than is widely believed.) Now scientists are describing another impact that has received relatively little attention but appears to also be profound: Dams block sediment-carrying river pulses into the ocean. [Continue reading…]