Vegetation is returning to this Elwha River valley in Washington state after two dams were removed.  OLYMPIC NATIONAL PARK/FLICKR

Why the World’s Rivers Are Losing Sediment and Why It Matters

Vast amounts of river-borne sediment are trapped behind the world’s large dams, depriving areas downstream of material that is badly needed to build up the marshes and wetlands that act as a buffer against rising seas.

By Jim Robbins