• mouth
    The Merrimack River's mouth has strong tidal currents.

    Photograph by Jack Fletcher

    World's Biggest Delta
    A delta is formed where a river empties into a larger body of water and deposits large amounts of alluvium. The biggest delta in the world is the Ganges Delta in Bangladesh and India. The delta is 350 kilometers (220 miles) along the Bay of Bengal. So many smaller rivers empty into the Ganges Delta, the area is sometimes called the "Mouths of the Ganges."

    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, knowing this, 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 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. Freshwater only reaches the Colorado River Delta when reservoirs created by man-made 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.

  • Term Part of Speech Definition Encyclopedic Entry
    abundant Adjective

    in large amounts.

    alluvium Noun

    gravel, sand, and smaller materials deposited by flowing water.

    bait fish Noun

    small fish used as food or lure for larger fish.

    bank Noun

    a slope of land adjoining a body of water, or a large elevated area of the sea floor.

    buffet Noun

    meal where guests serve themselves from a variety of different foods.

    city Noun

    large settlement with a high population density.

    clay Noun

    type of sedimentary rock that is able to be shaped when wet.

    Colorado delta clam Noun

    endangered species native to the mouth of the Colorado River in North America.

    cottonwood Noun

    North American tree with fuzzy seeds.

    current Noun

    steady, predictable flow of fluid within a larger body of that fluid.

    Encyclopedic Entry: current
    dam Noun

    structure built across a river or other waterway to control the flow of water.

    debris Noun

    remains of something broken or destroyed; waste, or garbage.

    delta Noun

    the flat, low-lying plain that sometimes forms at the mouth of a river from deposits of sediments.

    Encyclopedic Entry: delta
    deposit Verb

    to place or deliver an item in a different area than it originated.

    devastate Verb

    to destroy.

    dynamic Adjective

    always changing or in motion.

    endanger Verb

    to put at risk.

    erode Verb

    to wear away.

    freshwater Adjective

    having to do with a habitat or ecosystem of a lake, river, or spring.

    gravel Noun

    small stones or pebbles.

    Great Lakes Noun

    largest freshwater bodies in the world, located in the United States and Canada. Lake Huron, Lake Ontario, Lake Michigan, Lake Erie, and Lake Superior make up the Great Lakes.

    habitat Noun

    environment where an organism lives throughout the year or for shorter periods of time.

    Encyclopedic Entry: habitat
    harbor Noun

    part of a body of water deep enough for ships to dock.

    Encyclopedic Entry: harbor
    hole Noun

    spot in a river where the water is still and deep.

    lack Noun

    absence or reduction.

    lake Noun

    body of water surrounded by land.

    mouth Noun

    place where a river empties its water. Usually rivers enter another body of water at their mouths.

    Encyclopedic Entry: mouth
    predator Noun

    animal that hunts other animals for food.

    prey Noun

    animal that is hunted and eaten by other animals.

    reservoir Noun

    natural or man-made lake.

    Encyclopedic Entry: reservoir
    river bed Noun

    material at the bottom of a river.

    river management Noun

    the art and science of controlling the flow, path, and power of rivers.

    sand Noun

    small, loose grains of disintegrated rocks.

    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
    silt Noun

    small sediment particles.

    Encyclopedic Entry: silt
    spawn Verb

    to give birth to.

    walleye Noun

    freshwater fish native to North American rivers.

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