High School Earth Science/Surface Water

As we've learned, some of the freshwater on the Earth is on the surface, in streams, rivers, ponds and lakes. This freshwater is tremendously important to humans, plants, and animals. Wetlands are areas where water bodies and land meet. Wetlands contain high biodiversity and play a key role in naturally removing pollutants from water. At times, surface waters flood, which often creates hazardous conditions for people on the ground.

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.

Streams and Rivers

A stream is a body of moving water confined by a bottom (or bed) and earthen sides (or banks). There are many categories of streams including creeks, brooks, tributaries, bayous, and rivers—all of these types of streams vary in their size, depth, speed, and location. Streams are always-changing natural objects where water flows downhill, taking turns through hills and plains as elevation, rock type, and topography guide the stream along. They are responsible for a great deal of erosion and can create great canyons over time, as they slowly move soil, pebbles, and even boulders downstream.

Parts of a Stream

The place at which a stream originates is called the source; this is often a spring but it could be the top of a mountain. When two streams come together, the point where they join is called a confluence. The smaller of the two streams is considered a tributary of the larger stream. A pool in a stream is somewhat like a swimming pool — it's a slow part in the stream where water moves more slowly, so that the stream spreads out and becomes deeper. Finally, the point at which a stream comes into a large body of water, like an ocean or a lake is called the mouth. These areas are called estuaries, and they oftentimes form unique ecosystems where water from the stream and the lake or ocean mix together (Figure 13.8).


Figure 13.8: (Left) This estuary at Damas Island, Costa Rica, shows how water, plants, and land all come together in an estuary. (Right) This is a satellite image of the Nile Delta, showing the unique ecosystem around the estuary, and its shape (the name delta comes from the Greek letter Δ).

Rivers

Rivers are the largest type of stream, and move large amounts of water through landscapes from higher to lower elevations. North America has several divides that separate the land up into separate water basins (Figure 13.9). In each of these sections, rivers will eventually run to the Atlantic Ocean, Pacific Ocean, the Great Lakes, Arctic Ocean, or the Gulf of Mexico. Most rivers are bordered by floodplains, which are flat areas that flood when rivers overflow their banks.

**Figure 13.9**: The divides of North America.

Rivers generally move a lot of water. The Amazon River, the world's river with the greatest flow, has a flow rate of nearly 220,000 cubic meters per second! By comparison, at Niagara Falls, nearly 1,800 cubic meters of water fall per second (Figure 13.10).

Since rivers contain so much water, humans have used them since the beginning of civilization as a source of water, food, transportation, defense, power, recreation, and waste disposal. The water you drink probably comes from a reservoir fed by rivers. The electricity in your house may also come from power plants that use rivers to generate power. Obviously, the natural areas along by rivers are affected by humans use or misuse of the rivers. Sometimes entire populations of organisms can be destroyed by pollution of a river many miles upstream. Table 13.1 shows the ten longest rivers in the world.

Table 13.1: Ten Longest Rivers in the World
#	Name	Continent	Rate of Flow (^m³⁄_s)	Approximate Length (km)
1	The Nile	Africa	2,900	6,695
2	The Amazon	South America	225,000	6,683
3	Yangtze	Asia	33,000	6,380
4	Mississippi River	North America	13,000	5,970
5	Ob River	Asia	13,000	5,410
6	Huang He	Asia	2,600	4,830
7	Congo	Africa	43,000	4,630
8	Lena	Asia	17,000	4,400
9	Amur	Asia	6,000	4,350
10	Yenisei River	Asia	20,000	4,106

Ponds and Lakes

Streams and rivers, by definition, are bodies of water that have a current; they are in constant motion. Ponds and lakes, on the other hand, do not (Figure 13.11). They are generally bordered by hills or low rises, so that the water is blocked from flowing directly downhill. They represent yet another important resource for humans and another area in need of conservation.

Though the word pond refers to water that does not constantly flow downhill, there is disagreement about the exact definition of a pond. It is generally agreed, however, that a pond is a small body of freshwater. You probably wouldn't need a boat to get across it, and you might be able to stand up in it. Little or no surface water would escape from the pond through streams, and they are often fed by underground springs.

Lakes are larger bodies of freshwater formed by some natural process like tectonic plate movement, landslides, or human actions, such as building a dam. Almost all lakes are freshwater, and water usually leaves the lake through a river or a stream. All lakes lose some water to evaporation.

Some lakes are so large that they have their own tidal systems and currents, and can affect weather patterns. The Great Lakes in the United States, for example, contain 22% of the world's fresh surface water (Figure 13.12). The largest of the Great Lakes, Lake Superior, has a tide that rises and falls several centimeters each day. The Great Lakes are large enough to change the entire weather system in the Northeast region of the United States, in what is known as the "lake effect". They are home to countless species of fish and wildlife as well.

Lakes can be formed in a variety of different ways. Some lakes, like The Great Lakes fill depressions eroded as glaciers scraped soil and rock out from the landscape. Lakes known as crater lakes, formed in volcanic calderas that have filled up with precipitation. Rift lakes are formed in cracks created by tectonic faults. And subglacial lakes are found below a frozen ice cap. As a result of geologic history and the arrangement of land masses on the Earth, most lakes are in the Northern Hemisphere. In fact, over 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.

Limnology is the study of all bodies of freshwater and the organisms that live there. The ecosystem of a lake is divided into three distinct sections (Figure 13.13):

The littoral zone, which is the sloped area closest to the edge of the water.
The open-water zone (also called the photic or limnetic zone), where sunlight is abundant.
The deep-water zone (also called the aphotic or profundal zone), where little or no sunlight can reach.

Much life is found in the littoral zone, because sunlight allows the growth of plants on the lake bed. These plants in turn, provide food and shelter to animals like snails, insects, and fish. Other plants and fish such as bass and trout live in the open-water zone. The deep-water zone does not allow for plants to grow, so fewer organisms live there. In this zone, most organisms are scavengers like crabs and catfish, which feed on dead organisms that fall to the bottom. Fungi and bacteria aid in the decomposition of those dead organisms, too. 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 always permanent features of a landscape. Some intermittent lakes come and go with the seasons, as water levels rise and fall. Over a longer time period, lakes can disappear when they are filled in with sediments, if the springs or streams that fill them diminish, or if their outlets grow due to erosion. When the climate of an area alters, lakes can either expand or shrink, and lakes may disappear altogether if precipitation significantly diminishes.

Wetlands

Types of Wetlands

A wetland is a land area that is saturated with water, either permanently or seasonally, such that it takes on the characteristics of a distinct ecosystem.

A swamp is a wetland characterized by lush trees and vines in a low-lying area beside slow-moving rivers (Figure 13.15).

**Figure 13.15**: A swamp is characterized by trees in still water.

An estuary is an area where saltwater from the sea mixes with freshwater from a stream or river (Figure 13.16). 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, which has more salt than freshwater but less than sea water. Because estuaries contain areas of water with many different levels of dissolved salt, they tend to have many different habitats for plants and animals. As a result, estuaries have extremely high biodiversity.

Ecological Role of Wetlands

As mentioned above, wetlands are home to many different species of organisms. Though they make up only 5% of the area of the United States, wetlands contain more than 30% of the plant types found in the United States. Many endangered species live in wetlands, and therefore many 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 rushed off a farmer's fields, and therefore prevent that fertilizer from contaminating another body of water. Since wetlands naturally purify water, preserving wetlands also helps to maintain clean supplies of water.

Primarily, the factor that distinguishes wetlands from other land forms or water bodies is the characteristic vegetation of aquatic plants, adapted to its unique hydric soil. Wetlands play a number of roles in the environment, principally water purification, flood control, and shoreline stability. Wetlands are also considered the most biologically diverse of all ecosystems, serving as home to a wide range of plant and animal life. Wetlands occur naturally on every continent except Antarctica, the largest including the Amazon River basin, the West Siberian Plain, and the Pantanal. The water found in wetlands can be freshwater, brackish, or saltwater.[4] The main wetland types include swamps, marshes, bogs, and fens; and sub-types include mangrove, carr, pocosin, and varzea. They can also be constructed artificially as a water management tool, which may play a role in the developing field of water-sensitive urban design. The UN Millennium Ecosystem Assessment determined that environmental degradation is more prominent within wetland systems than any other ecosystem on Earth. International conservation efforts are being used in conjunction with the development of rapid assessment tools to inform people about wetland issues.

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 different ways, and their effects can be minimized in several different ways. Perhaps unsurprisingly, floods tend to affect low-lying areas most severely.

Causes of Floods

Floods usually occur when precipitation occurs more quickly than that water can be absorbed into the ground or carried away by rivers or streams. Flooding may be sudden and unexpected, in the case of a flash flood, when very intense rainfall occurs in an area (Figure 13.17). This strong rainfall will fall too fast to be absorbed into the ground, and will overflow the banks of the streams and rivers. Alternately, floods can occur more slowly, when a long period of rainfall fills the ground with water and the levels of rivers and streams gradually rises. Less commonly, floods can occur when a dam breaks along a reservoir—as you might expect, this type of flooding can be catastrophic. In California, floods commonly occur when rainfall far exceeds annual averages, such as during an El Niño year. High water levels have also caused small dams to break, wreaking havoc downstream.

Vegetation is an important factor in determining whether a flood occurs. Plants tend to slow down the water that 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 across the earth, increasing the time between rainfall and the water's arrival in a stream. For the same reason, wetlands also play a key role in minimizing the impacts of floods; they act as a buffer between land and high water levels. Flooding is therefore less common in areas that are heavily vegetated, and can be more severe in areas that have been recently logged.

Effects of Floods

The most recent catastrophic flooding in United States history occurred in New Orleans in 2005, in the aftermath of Hurricane Katrina (Figure 13.18). This flooding occurred because of the failure of the city's levees, raised structures designed to hold back a river or lake. The New Orleans levees were poorly designed, and broke in multiple places after the storm, allowing water to pour into the city (Figure 13.19). Ultimately, over 80% of the city was submerged, 90% of city residents evacuated, and over 1800 people died in the disaster.

**Figure 13.19**: Levees that held back flood water were broken in many key areas around the city of New Orleans.

Not all the consequences of flooding are negative. Floods deposit sediment in their floodplains, and these sediments are nutrient rich and good for farming. Therefore, many farmers today grow crops in the floodplains of major rivers. This pattern of rain and flooding was also important to such peoples as the ancient Egyptians along the Nile River.

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. Many plants and fish along the Colorado River, for example, depend on seasonal flooding to rearrange sand bars.

Lesson Summary

One way water returns to the oceans is through rivers and streams.
Streams begin in higher elevations, with many tributaries joining together as it flows to lower elevations.
A mature river will develop a floodplain and may eventually form a delta where the river meets the ocean.
Water temporarily resides in ponds and lakes, which are mostly freshwater.
Scientists study lakes, wetlands, and estuaries because they are biologically important areas.
Flooding is part of the natural cycle of all rivers, which enriches floodplains with important nutrients.
Flooding produces difficulties for humans living on or near the floodplain and in coastal areas, particularly when levees break.

Review Questions

Where do streams originate?
Compare and contrast streams and rivers.
What is an advantage and disadvantage of living in floodplains?
Which of the ten longest rivers has the greatest rate of flow?
Compare and contrast ponds and lakes.
What are three main types of wetlands?
Consider an animal common in swamps and an animal common in rivers. What natural adaptations do they each have to their habitat?
Deserts are places that get little rain. Why are they in danger of flash floods at times?

Vocabulary

aphotic zone: The region in a freshwater body where no sunlight can reach. Also called the deep-water zone or the profundal zone.
brackish: Water that is a mixture of freshwater and saltwater.
confluence: The point where two streams join together.
divide: A ridge that separates one water basin from another. Each water basin will be drained by streams into a different ocean.
estuary: An area where saltwater from the sea mixes with freshwater from a stream or river. Estuaries often have high biodiversity.
floodplain: A flat area covered by a stream or river when it floods. Often rich in nutrients and thus good places to farm.
intermittent lake: A lake that appears and disappears seasonally, as water levels rise and fall.
lake: A larger body of freshwater, usually drained by a stream. May be naturally occurring or humanmade.
levee: A raised structure designed to hold back the waters of a stream or river in the case of a flood.
limnology: The study of all freshwater bodies and the organisms that live in them.
littoral zone: The region in a freshwater body closest to shore. Usually contains the most life in the body of water.
marsh: A type of wetland around lakes, streams, or the ocean where grasses and reeds are common, but there are no trees. May be freshwater, saltwater, or brackish. Water is generally shallow.
mouth: The point where a stream enters a larger body of water like a lake or an ocean.
photic zone: The region in a freshwater body where sunlight is abundant. Also called the open-water zone or limnetic zone.
pond: A small body of freshwater, with no stream draining it. Often fed by an underground spring.
pool: A deep, slow-moving part of the stream. The stream is usually wider at the point where a pool is found.
source: The place where a stream starts.
stream: A body of moving water, contained within a bank (sides) and bed (bottom).
swamp: A wetland in a low-lying area, where water moves very slowly. Oxygen levels are often low in swamps.
tributary: The smaller of two streams that join together to make a larger stream.
wetland: A region of land that holds a great deal of water for significant periods of time, and that contains specialized plants able to grow in these wet conditions.

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?

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