3.  After the architect decides where to place the building on the site, she must consider erosion. Before building can begin, the trees will be cut down. All the plants will be scraped away by bulldozers. Water is a major cause of erosion, but plants hold soil in place. After the vegetation is gone from the site, rain will cause the soil to move downhill. Using the topographic map, the architect will be able to predict which direction the soil will move. She will tell the builder where to place fences to help stop erosion. This is why the information from the topographic map is valuable.

4.  The plastic fences around building sites are called silt fences. Silt is a very fine, small sediment. These fences are placed at the bottom of the hill or along a roadway. The silt fences keep the eroding soil from going into a waterway, such as a stream or into the drainage system of a city or town. If silt gets into a stream, sediment deposition can dam up the stream and cause flooding upstream and loss of water downstream.

5.  Look at these two satellite photographs of the Pareechu River in Tibet. You will see what happens when erosion happens on a large scale. The top picture shows the Pareechu River after it has been blocked. Can you see the landslide in the lower right corner? This landslide was caused when heavy rains loosened the soil and rock at the top of a hill. Gravity caused them to slide down into the river. The large blue area is a landslide lake that was formed after the landslide blocked the river. You can also see how wide the river has become.

Mapping Erosion Control

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6.E 6.1 Changing Earth's Surface

1.  Have you ever noticed that plastic fencing around a building site and wondered why it’s there? The answer is erosion control. Before an architect begins to design a building, she must conduct a site survey. One of the steps in a site survey is to look at the elevations of the building site and surrounding areas.

2.  Elevations are found on topographic maps. Topographic maps show large and small hills, streams, rivers, valleys, and man- made features. This information helps the architect decide where to build the structure on the site. It would not be a good idea to place a structure near a river that could erode away all of the sediment from under the building. The architect also has to consider where people will park their cars. If the land is too steep, the cars could roll down the hill! Much of the information needed to properly design the building site is shown on a topographic map.

6.  The bottom photograph was taken almost a year later. The water has started to drain out of the lake. The lake has gotten smaller, and the river behind it is not as wide. It is obvious that the land around the lake has hills and valleys. Satellite photographs like these give a clear picture of different erosional features such as landslides and deltas. These pictures allow scientists to study the features for changes over time.

7.  Scientists can use topographic maps together with satellite views to gain vast amounts of information. Topographic maps don’t change much unless there is a catastrophic natural disaster, such as when a volcano explodes. During a catastrophic event, satellite pictures can change daily. A satellite view can show the visual difference as seasons change. Satellite photographs show how much vegetation is present from one season to the next. This allows scientists to predict how well this vegetation will prevent erosion when rains start. Scientists can lay the satellite view of the vegetation over the topographic map and predict erosional patterns. This allows cities and towns to undertake erosion control measures. They may build retaining walls or plant more vegetation to prevent landslides from occurring.

8.  In the past, topographers (map makers) had to travel to the area they were mapping to create a topographic map. Now using satellites, scientists have been able to generate a complete digital topographic map of over 99% of Earth. The map was created using measurements from an instrument called ASTER, on NASA’s Terra satellite. This technology is much more accurate than the original way of creating topographic maps.

9.  Being able to read topographic maps is a skill required in many careers. Geologists, architects, city planners, and engineers must all have this skill. Whether they are reading the older paper topographic maps or the newer digital topographic maps, these tools allow them to do their jobs well.

6.E 6.1 Changing Earth's Surface

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1 How does a topographic map help an architect?

A It shows where to build a structure.

B It shows how to build a structure.

C It shows the elevation of a building site.

D It tells the builder how the architect wants the parking lot.

2 How do plants help in erosion control?

A Plant roots hold the soil in place.

B Plants are pretty and keep the neighborhood from being ugly.

C Plants don't help in erosion control.

D Plants allow the soil to float in the rainwater.

3 The term deposition is used in paragraph 4. Based on context, this term means ____________.

A laying down of sediments

B erosion control

C silt fences

D landslides

6.E 6.1 Changing Earth's Surface

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4 How do satellite views help scientists?

A They give scientists the ability to predict catastrophic events.

B Satellite cameras are not good enough to really be helpful.

C They allow scientists to know where valuable minerals are located.

D They allow scientists to see changes as they happen.

6.E 6.1 Changing Earth's Surface

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5 These diagrams are topographic maps of Mount St. Helens in Washington state. They were made before and after the volcano erupted in 1980. What do these two maps tell us about the eruption?

A The eruption melted a glacier and caused massive flooding.

B The eruption blew out the north wall of the volcano's cone.

C The eruption destroyed most of the surrounding forest.

D The eruption did not change the shape of the volcano.