Lesson Objectives

• Compare streams and rivers and their importance.
• Describe what ponds and lakes are, and why they are important.
• Explain why wetlands are significant in the water cycle, and describe their biodiversity.
• Describe the causes of floods and their effects.

Vocabulary

• brackish
• confluence
• continental divide
• divide
• estuary
• flood
• lake
• levee
• limnology
• marsh
• mouth
• pond
• pool
• stream
• swamp
• tributary
• wetland

Introduction

Fresh water in streams, ponds, and lakes is an extremely important part of the water cycle if only because of its importance to living creatures. Along with wetlands, these fresh water regions contain a tremendous variety of organisms.

Streams and Rivers

Streams are bodies of water that have a current; they are in constant motion. Geologists recognize many categories of streams depending on their size, depth, speed, and location. Creeks, brooks, tributaries, bayous, and rivers might all be lumped together as streams. In streams, water always flows downhill, but the form that downhill movement takes varies with rock type, topography, and many other factors. Stream erosion and deposition are extremely important creators and destroyers of landforms and were described in the Erosion and Deposition chapter.

Parts of a Stream

A stream originates at its source. A source is likely to be in the high mountains where snows collect in winter and melt in summer, or a source might be a spring. A stream may have more than one source.

Two streams come together at a confluence. The smaller of the two streams is a tributary of the larger stream (Figure below).

A stream may create a pool where water slows and becomes deeper (Figure below).

The point at which a stream comes into a large body of water, like an ocean or a lake is called the mouth. Where the stream meets the ocean or lake is an estuary (Figure below).

The mix of fresh and salt water where a river runs into the ocean creates a diversity of environments where many different types of organisms create unique ecosystems (Figure below).

Rivers

Rivers are the largest types of stream, moving large amounts of water from higher to lower elevations (Figure below). The Amazon River, the world’s river with the greatest flow, has a flow rate of nearly 220,000 cubic meters per second!

People have used rivers since the beginning of civilization as a source of water, food, transportation, defense, power, recreation, and waste disposal.

Divides

A divide is a topographically high area that separates a landscape into different water basins (Figure below). Rain that falls on the north side of a ridge flows into the northern drainage basin and rain that falls on the south side flows into the southern drainage basin. On a much grander scale, entire continents have divides, known as continental divides.

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 fresh water that usually have no outlet (Figure below); ponds are often are fed by underground springs.

Lakes are larger bodies of water. Lakes are usually fresh water, 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% of the world’s fresh surface water (Figure below). 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 weather system in Northeastern United States 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 a variety of different ways: in depressions carved by glaciers, in calderas (Figure below), and along tectonic faults, to name a few. Subglacial lakes are even found below a frozen ice cap.

As a result of geologic history and the arrangement of land masses, most lakes are in the Northern Hemisphere. In fact, more than 60% of all the world’s lakes are in Canada — most of these lakes were formed by the glaciers that covered most of Canada in the last Ice Age (Figure below).

Limnology is the study of bodies of fresh water and the organisms that live there. The ecosystem of a lake is divided into three distinct sections (Figure below):

1. The surface (littoral) zone is the sloped area closest to the edge of the water.

2. The open-water zone (also the photic or limnetic zone) has abundant sunlight.

3. The 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 (Figure below). Lakes may disappear if precipitation significantly diminishes.

Wetlands

Wetlands are lands that are wet for significant periods of time. They are common where water and land meet. Wetlands can be large flat areas or relatively small and steep areas.

Wetlands are rich and unique ecosystems with many species that rely on both the land and the water for survival. Only specialized plants are able to grow in these conditions. Wetlands tend have a great deal of biological diversity. Wetland ecosystems can also be fragile systems that are sensitive to the amounts and quality of water present within them.

Types of Wetlands

Marshes are shallow wetlands around lakes, streams, or the ocean where grasses and reeds are common, but trees are not (Figure below). Frogs, turtles, muskrats, and many varieties of birds are at home in marshes.

A swamp is a wetland with lush trees and vines found in a low-lying area beside slow-moving rivers (Figure below). Like marshes, they are frequently 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 fresh water, salt water, or a mixture of both.

In an estuary, salt water from the sea mixes with fresh water from a stream or river (Figure below). 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 creates. Estuaries contain brackish water, water that has more salt than fresh water but less than sea water. Because of the rapid changes in salt content, estuaries have many different habitats for plants and animals and extremely high biodiversity.

Ecological Role of Wetlands

As mentioned above, wetlands are home to many different species of organisms. Although they make up only 5% of the area of the United States, wetlands contain more than 30% of the plant types. Many endangered species live in wetlands, so wetlands are protected from human use.

Wetlands also play a key 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 helps to maintain clean supplies of water.

Floods

Floods are a natural part of the water cycle, but they can be terrifying forces of destruction. Put most simply, a flood is an overflow of water in one place. Floods can occur for a variety of reasons, and their effects can be minimized in several different ways. Perhaps unsurprisingly, floods tend to affect low-lying areas most severely.

“Floods 101” is a National Geographic video found in Environment Video, Natural Disasters, Landslides, and more: http://video.nationalgeographic.com/video/player/environment/

Causes of Floods

Floods usually occur when precipitation falls more quickly than that water can be absorbed into the ground or carried away by rivers or streams. Waters may build up gradually over a period of weeks, when a long period of rainfall or snowmelt fills the ground with water and raises stream levels.

Extremely heavy rains across the Midwestern U.S. in April 2011 led to flooding of the rivers in the Mississippi River basin in May 2011 (Figures below and below).

Flash floods are sudden and unexpected, taking place when very intense rains fall over a very brief period (Figure below). A flash flood may do its damage miles from where the rain actually falls if the water travels far down a dry streambed so that the flash flood occurs far from the location of the original storm.

Heavily vegetated lands are less likely to experience flooding. Plants slow down water as it runs over the land, giving it time to enter the ground. Even if the ground is too wet to absorb more water, plants still slow the water’s passage and increase the time between rainfall and the water’s arrival in a stream; this could keep all the water falling over a region to hit the stream at once. Wetlands act as a buffer between land and high water levels and play a key role in minimizing the impacts of floods. Flooding is often more severe in areas that have been recently logged.

When a dam breaks along a reservoir, flooding can be catastrophic. High water levels have also caused small dams to break, wreaking havoc downstream.

People try to protect areas that might flood with dams, and dams are usually very effective. People may also line a river bank with levees, high walls that keep the stream within its banks during floods. A levee in one location may just force the high water up or downstream and cause flooding there. The New Madrid Overflow in the Figure above was created with the recognition that the Mississippi River sometimes simply cannot be contained by levees and must be allowed to flood.

Effects of Floods

Not all the consequences of flooding are negative. Rivers deposit new nutrient-rich sediments when they flood and so floodplains have traditionally been good for farming. Flooding as a source of nutrients was important to Egyptians along the Nile River until the Aswan Dam was built in the 1960s. Although the dam protects crops and settlements from the annual floods, farmers must now use fertilizers to feed their crops.

Floods are also responsible for moving large amounts of sediments about within streams. These sediments provide habitats for animals, and the periodic movement of sediment is crucial to the lives of several types of organisms. Plants and fish along the Colorado River, for example, depend on seasonal flooding to rearrange sand bars.

Lesson Summary

• Streams return water to the oceans.
• Stream headwaters are at higher elevations where snow melts or where there are springs.
• Tributaries join together as a river flows to its mouth at lower elevations.
• A river may eventually form a delta with an estuary where it meets the ocean.
• Water temporarily resides in ponds and lakes, which are mostly fresh water.
• Flooding is part of the natural cycle of rivers, enriching floodplains with nutrients, but flooding may destroy crops and settlements.

Review Questions

1. Where do streams originate?

2. Compare and contrast streams and rivers.

3. What is an advantage and disadvantage of living in floodplains?

4. Compare and contrast ponds and lakes.

5. What are wetlands and what is the value of wetlands?

6. Consider an animal common in swamps and an animal common in rivers. What natural adaptations do they each have to live in their habitat?

7. Deserts get little rain so why are they in danger of flash floods?

Points to Consider

• What types of streams have you seen in your area?
• Why are bodies of water never really permanent?
• Is it possible that your home could be flooded? What would you do if it were flooded?