The place where a river enters a lake, larger river, or the ocean is called its mouth. River mouths are places of much activity.

As a river flows, it picks up sediment from the river bed, eroding banks, and debris on the water. The river mouth is where much of this gravel, sand, silt, and clay—called alluvium—is deposited.

When large amounts of alluvium are deposited at the mouth of a river, a delta is formed. The river slows down at the mouth, so it doesn’t have the energy to carry all the silt, sand, and clay anymore. These sediments form the flat, usually triangle-shaped land of a delta. Examples of deltas are the Nile River Delta in Egypt and the Mississippi River Delta in the U.S. state of Louisiana.

The mouth of a river is often a good place for fishing. Along with the alluvium, a river flushes many different species into the lake or sea. Larger fish sometimes wait at the mouth of the river for an easy meal. Thanks to the current of the river, the large fish have a “buffet” of smaller bait fish. This meeting of big and small fish means there is more for people to catch.

But fish can hide at river mouths, too. The current changes here, and fish can find a “hole,” as fishermen call it, with relatively calm water. Smaller fish hide from larger predators and look for new sources of food. In the Great Lakes area of North America, for instance, walleye (Sander vitreus) take advantage of the holes. Walleye leave their spawning locations in a river to look for food. A rushing river gives way to calm spots as it enters a lake, and walleye can rest there without being carried into the lake. Fishermen cast their line into these undisturbed waters, knowing they’re a favorite spot for walleye.

The destruction of a river’s mouth can devastate the surrounding area. The Colorado River naturally flows into the Sea of Cortez, in Mexico. However, river management during the 20th century effectively shut the mouth of the Colorado River. Fresh water only reaches the Colorado River Delta when reservoirs created by artificial dams are full. Native species, such as cottonwood trees and the Colorado Delta clam, are endangered because of their lack of habitat.

Many major port cities have been built at river mouths. The abundant wildlife and natural transportation often create dynamic harbors and ports. Rotterdam, a large city in the Netherlands, is at the mouth of the Rhine River. It is one of the busiest port cities in the world. Pisa, Italy, home to the Leaning Tower of Pisa, is at the mouth of the Arno River.