Fluvial Processes and Systems

6.3 Wetlands

Wetlands are lands that are wet for significant periods. They are common where water and land meet. Wetlands can be large flat areas or small and steep areas. Wetlands are vibrant and unique ecosystems with many species that rely on both the land and the water for survival. Only specialized plants can grow in these conditions. Wetlands tend to have a great deal of biological diversity. Wetland ecosystems can also be fragile systems sensitive to the amounts and quality of water present within them. Learn more about wetlands further from the U.S. Environmental Protection Agency.

Sunrise at Viru Bog, Estonia” is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported license.

Types of Wetlands

There are a variety of distinct types of wetlands. Marshes are shallow wetlands around lakes, streams, or the ocean where grasses and reeds are common, but trees are not. Frogs, turtles, muskrats, and many varieties of birds are at home in marshes.

Bride Book Salt Marsh” is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported license.

A swamp is a wetland with lush trees and vines found in a low-lying area beside slow-moving rivers. Like marshes, they are often or always inundated with water. Since the water in a swamp moves slowly, oxygen in the water is often scarce. Swamp plants and animals must be adapted for these low-oxygen conditions. Like marshes, swamps can be freshwater, saltwater, or a mixture of both.

National Wildlife Refuge, Georgia” by the United States Fish and Wildlife Service is licensed as Public Domain.

In an estuary, salt water from the sea mixes with fresh water from a stream or river. These semi-enclosed areas are home to plants and animals that can tolerate the sharp changes in salt content that the constant motion and mixing of waters create. Estuaries contain brackish water, water that has more salt than freshwater but less than seawater. Because of the rapid changes in salt content, estuaries have many different habitats for plants, animals, and extremely high biodiversity.

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Ecological Role of Wetlands

As mentioned above, wetlands are home to many varied species of organisms. Although they make up only 5 percent of the United States area, wetlands contain more than 30 percent of the plant types. Many endangered species live in wetlands, so wetlands are protected from human use. Wetlands also offer protection from storm surges from tropical storm systems like hurricanes and supply safe, hidden protection for hatchlings.

Wetlands also play a critical biological role by removing pollutants from water. For example, they can trap and use fertilizer that has washed off a farmer’s field, and therefore they prevent that fertilizer from contaminating another body of water. Since wetlands naturally purify water, preserving wetlands also help to maintain clean supplies of water.

Ponds and Lakes

Ponds and lakes are bordered by hills or low rises so that the water is blocked from flowing directly downhill. Ponds are small bodies of freshwater that usually have no outlet; ponds are often are fed by underground springs. Lakes are more substantial bodies of water. Lakes are usually freshwater, although the Great Salt Lake in Utah is just one exception. Water usually drains out of a lake through a river or a stream, and all lakes lose water to evaporation.

Large lakes have tidal systems and currents, and can even affect weather patterns. The Great Lakes in the United States contain 22 percent of the world’s fresh surface water. The largest them, Lake Superior, has a tide that rises and falls several centimeters each day. The Great Lakes are large enough to alter the Northeastern United States’ weather system by the “lake effect,” which is an increase in snow downwind of the relatively warm lakes. The Great Lakes are home to countless species of fish and wildlife.

Lakes form in various ways: in depressions carved by glaciers, in calderas, and along tectonic faults, to name a few. Subglacial lakes are even found below an ice cap. As a result of geologic history and landmasses’ arrangement, most lakes are in the Northern Hemisphere.

Limnology is the study of bodies of freshwater and the organisms that live there. The ecosystem of a lake is divided into three distinct sections:

  • Surface (littoral) zone is the sloped area closest to the edge of the water.
  • Open-water zone (also the photic or limnetic zone) has abundant sunlight.
  • Deep-water zone (also the aphotic or profundal zone) has little or no sunlight.

There are several life zones found within a lake. In the littoral zone, sunlight promotes plant growth, which provides food and shelter to animals such as snails, insects, and fish. In the open-water zone, other plants and fish, such as bass and trout, live. The deep-water zone does not have photosynthesis since there is no sunlight. Most deep-water organisms are scavengers, such as crabs and catfish that feed on dead organisms that fall to the bottom of the lake. Fungi and bacteria aid in the decomposition in the deep zone. Though different creatures live in the oceans, ocean waters also have these same divisions based on sunlight with similar types of creatures that live in each of the zones.

Lakes are not permanent features of a landscape. Some come and go with the seasons, as water levels rise and fall. Over a longer time, lakes disappear when they fill with sediments, if the springs or streams that fill them diminish, or if their outlets grow because of erosion. When the climate of an area changes, lakes can either expand or shrink. Lakes may disappear if precipitation significantly diminishes.


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Physical Geography and Natural Disasters by R. Adam Dastrup, MA, GISP is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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