Download 7.2: Moving water shapes land

Chapter 7: Erosion
and Deposition
7.1: Forces wear down and build up Earth’s surface
7.2: Moving water shapes land
7.3: Waves and wind shape land
7.4: Glaciers carve land and move sediments
7.2 Moving water shapes land
Before, you learned:
Erosion is the movement of rock and soil
Gravity causes mass movements of rock and
soil
Now, you will learn:
How moving water shapes Earth’s surface
How water moving under ground forms caves
and other features
Streams shape Earth’s surface
 Think about a river:
Surrounding land is higher than the river
Steep valley: see the river is the low point
Even in very flat places, land is sloping down to the
river, which is running downhill through the land
 Water running downhill changes the land:
Shapes landforms by moving sediment in the processes
of erosion and deposition
 Water can carve out new features underground
Drainage Basins and Divides
 Where does water go when it rains or ice melts
on a slope?
Soaks in the ground
flows down the slope in thin sheets
Becomes part of a channel that forms a stream
Stream: any body of water that flows down a slope
along a channel
 Streams flow into one another to form complex
drainage systems
Small flow into larger
Drainage Basin: area of land in which water drains into a
stream system
 Water usually eventually drains into a lake or ocean
Drainage Basins and Divides
Drainage basins are separated by ridges
called divides
A ridge from which water drains to one side or
the other
Can be along a mountain, or simply the highest
line of land
Are boarders of drainage basins
The Continental Divide
Runs from Alaska to
Mexico
Most water that falls
west: drains to the
Pacific Ocean
Most water that falls
east: drains into the Gulf
of Mexico or the Atlantic
Ocean
Valleys and Floodplains
 As streams flow and carry away sediment, they form valleys
 In high mountains they often cut V-shaped valleys that are narrow
and steepwalled
 In low areas, they may form broad valleys that include floodplains
 An area of land on either side of a stream that is underwater when
the stream floods
 Can make the land very fertile when it deposits sediments
Stream Channels
 Meanders: twisting
and looping pattern
in a stream channel
Erodes the outside
banks and deposits
sediment along the
inside banks
A flood can cause a
new channel to form,
cutting off the
meander into a
crescent-shaped
lake: oxbow lake
Alluvial Fans and Deltas
 Streams also create landforms
called alluvial fans and deltas,
formed by deposition of sediment
Alluvial fan: fan-shaped deposit of
sediment at the base of a mountain
 Forms where a stream leaves a steep
valley and enters a flatter plain
 Stream slows down and spreads out,
depositing sediment
Delta: area of land formed by the buildup
of sediment at the end, or mouth, of a
river
 When the river enters the ocean, it slows,
depositing sediment
• Sediment gradually builds up to form a plain
Water moving underground forms
caverns
Some rainwater runs into surface streams,
some evaporates, some is absorbed by
plants…and some soaks into the ground
and becomes groundwater
Deep below, spaces in soil and rock
become completely filled with water
The top of this region is called the “water table”
Below the water table is “groundwater”
Can change with rainfall
Groundwater
 Can cause erosion deep below where it slowly flows
through underground beds of rock and soil
 Rainwater is slightly acidic and can dissolve certain
rocks, such as limestone
 The groundwater can dissolve some of the limestone
and carry it away
 Can form caves
 Large caves = caverns
 If the water table drops, the cavern can be filled with air
 Cavern System of passageways, containing lakes and streams
 Sinkhole: a basin that forms when the roof of a cave becomes so
thin that it suddenly falls in
 Possibly because water that supported the roof drained away
http://www.pbs.org/wnet/nature/lessons/br
eaking-it-down/video-segments-violenthawaii/1702/ - hawaii
http://www.youtube.com/watch?v=IR6evrt
7els - bill nye
http://dsc.discovery.com/videos/planetearth-caves-limestone-erosion.html
http://dsc.discovery.com/videos/earth-liverain-and-floods/
http://www.cbsnews.com/video/watch/?id=
3440105n