13.2 Surface Water

Lesson Objectives

• Identify features of streams and rivers.
• Describe ponds and lakes and how they form.
• Explain why wetlands are important.
• State how floods occur.

Vocabulary

• flood
• lake
• pond
• river
• stream
• wetland

Introduction

Only a very small percentage of Earth’s water is fresh, liquid water on the surface. But that tiny fraction of water is vital. It is needed by humans, plants, and many other living things. Liquid freshwater flows over Earth’s surface in streams and rivers. It also forms ponds, lakes, and wetlands. People use freshwater for drinking, washing, and industry. They also use it for fun. How do you use freshwater for fun?

Streams and Rivers

Look at the pictures of flowing water in Figure below. A waterfall tumbles down a mountainside. A brook babbles through a forest. A river slowly meanders through a broad valley. What do all these forms of flowing water have in common? They are all streams.

All these forms of flowing water are streams.

What Are Streams and Rivers?

A stream is a body of freshwater that flows downhill in a channel. The channel of a stream has a bottom, or bed, and sides called banks. Any size body of flowing water can be called a stream. Usually, though, a large stream is called a river.

Features of Streams and Rivers

All streams and rivers have several features in common. These features are shown in (Figure below). The place where a stream or river starts is its source. The source might be a spring, where water flows out of the ground. Or the source might be water from melting snow on a mountain top. A single stream may have multiple sources. A stream or river probably ends when it flows into a body of water, such as a lake or an ocean. A stream ends at its mouth. As the water flows into the body of water, it slows down and drops the sediment it was carrying. The sediment may build up to form a delta.

Water in a stream flows along the ground from higher to lower elevation. What force causes the water to keep flowing?

Several other features of streams and rivers are also shown in Figure above.

• Small streams often flow into bigger streams or rivers. The small streams are called tributaries. A river and all its tributaries make up a river system.
• At certain times of year, a stream or river may overflow its banks. The area of land that is flooded is called the floodplain. The floodplain may be very wide where the river flows over a nearly flat surface.
• A river flowing over a floodplain may wear away broad curves. These curves are called meanders.

River Basins and Divides

All of the land drained by a river system is called its basin, or watershed. One river system’s basin is separated from another river system’s basin by a divide. The divide is created by the highest points between the two river basins. Precipitation that falls within a river basin always flows toward that river. Precipitation that falls on the other side of the divide flows toward a different river.

Figure below shows the major river basins in the U.S. You can watch an animation of water flowing through a river basin at this link: http://trashfree.org/btw/graphics/watershed_anim.gif

River basins in the U.S.

Ponds and Lakes

After a heavy rain, you may find puddles of water standing in low spots. The same principle explains why water collects in ponds and lakes. Water travels downhill, so a depression in the ground fills with standing water. A pond is a small body of standing water. A lake is a large body of standing water. Most lakes have freshwater, but a few are salty. The Great Salt Lake in Utah is an example of a saltwater lake.

The water in a large lake may be so deep that sunlight cannot penetrate all the way to the bottom. Without sunlight, water plants and algae cannot live on the bottom of the lake. That’s because plants need sunlight for photosynthesis.

The largest lakes in the world are the Great Lakes. They lie between the U.S. and Canada, as shown in Figure below. How great are they? They hold 22 percent of all the world’s fresh surface water!

The Great Lakes of North America get their name from their great size.

Water in Ponds and Lakes

Ponds and lakes may get their water from several sources. Some falls directly into them as precipitation. Some enters as runoff and some from streams and rivers. Water leaves ponds and lakes through evaporation and also as outflow.

How Lakes Form

The depression that allows water to collect to form a lake may come about in a variety of ways. The Great Lakes, for example, are glacial lakes. A glacial lake forms when a glacier scrapes a large hole in the ground. When the glacier melts, the water fills the hole and forms a lake. Over time, water enters the lake from the sources mentioned above as well.

Other lakes are crater lakes or rift lakes, which are pictured in Figure below. Crater lakes form when volcanic eruptions create craters that fill with water. Rift lakes form when movements of tectonic plates create low places that fill with water.

Craters and rifts become lakes when they fill with water. Where does the water come from?

Wetlands

Some of Earth’s freshwater is found in wetlands. A wetland is an area that is covered with water, or at least has very soggy soil, during all or part of the year. Certain species of plants thrive in wetlands, and they are rich ecosystems. Freshwater wetlands are usually found at the edges of steams, rivers, ponds, or lakes. Wetlands can also be found at the edges of seas.

Types of Freshwater Wetlands

Not all wetlands are alike, as you can see from Figure below. Wetlands vary in how wet they are and how much of the year they are soaked. Wetlands also vary in the kinds of plants that live in them. This depends mostly on the climate where the wetland is found. Types of wetlands include marshes, swamps, and bogs.

• A marsh is a wetland that is usually under water. It has grassy plants, such as cattails.
• A swamp is a wetland that may or may not be covered with water but is always soggy. It has shrubs or trees.
• A bog is a wetland that has soggy soil. It is generally covered with mosses.

These are just three of many types of wetlands.

Importance of Wetlands

People used to think that wetlands were useless. Many wetlands were filled in with rocks and soil to create lands that were then developed with roads, golf courses, and buildings. Now we know that wetlands are very important. Laws have been passed to help protect them. Why are wetlands so important?

• Wetlands have great biodiversity. They provide homes or breeding sites to a huge variety of species. Because so much wetland area has been lost, many of these species are endangered.
• Wetlands purify water. They filter sediments and toxins from runoff before it enters rivers, lakes, and oceans.
• Wetlands slow rushing water. During hurricanes and other extreme weather, wetlands reduce the risk of floods.

Although the rate has slowed, wetlands are still being destroyed today.

Floods

A flood occurs when so much water enters a stream or river that it overflows its banks. Flood waters from a river are shown in Figure below. Like this flood, many floods are caused by very heavy rains. Floods may also occur when deep snow melts quickly in the spring.

A river in Indiana floods after very heavy rains. Some areas received almost a foot of rain in less than 24 hours!

Floods are a natural part of the water cycle, but they can cause a lot of damage. Farms and homes may be lost, and people may die. In 1939, millions of people died in a flood in China. Although freshwater is needed to grow crops and just to live, too much freshwater in the same place at once can be deadly.

Lesson Summary

• A stream is a body of water that flows downhill in a channel. A large stream is usually called a river.
• Standing freshwater forms ponds and lakes. Lakes are generally bigger than ponds. Lakes may form in several different ways.
• A wetland is an area that is wet for all or part of the year. Wetlands are home to certain types of plants.
• Wetlands are very important. They have great biodiversity. They purify water. They slow down rushing water and help prevent floods.
• Floods occur when so much water enters a stream or river that it overflows its banks. Floods may be caused by heavy rains or melting snow. They can cause a lot of damage and loss of life.

Lesson Review Questions

Recall

1. What are the source and mouth of a river?

2. Define tributary and river system.

3. How may water enter a pond or lake?

4. What is a wetland?

5. List three reasons why wetlands should be protected.

Apply Concepts

6. For each stream pictured in Figure above, explain where it might be located on the map in Figure above.

Think Critically

7. The Nile River in Egypt empties into the Mediterranean Sea. At the mouth of the river, there is a very large delta. Explain how the delta formed.

8. Compare and contrast glacial, crater, and rift lakes.

Points to Consider

• In the desert, water runs in channels after a storm. The channels are dry otherwise. Is this a stream?
• It may seem hard to believe, but most of Earth’s freshwater is under our feet. It is stored below the surface of the ground.
  
  • How do you think water gets under the ground?
  • What happens to water after it goes under the ground? Is it trapped there forever?