What does the fact that 22 of the world's 32 largest cities stand on estuaries suggest?

Where a river meets the sea, fresh water flowing down from the land mixes with salty seawater. The result is brackish water, and the bodies of water where this happens are called estuaries. Similar places are found along large lakes, where river water mixes with lake water.

Estuaries are among the most productive natural places in the world. They serve as nurseries for young fish: about three-quarters of the fish that people catch depend on them. Many birds and other animals feed and breed in estuaries, and stop there to rest on their long migrations.

Estuaries also work for us in quieter ways. The marshes and sea grasses that grow in them filter the water that flows out to the ocean, and estuaries act as a buffer, protecting the coast from storms.

People have always been drawn to estuaries, for food, for work, for recreation and for the shelter they give. Of the 32 largest cities in the world, 22 stand on estuaries.