Water Erosion

Chapter 8 Section 2

Runoff and Erosion• Moving water is the major agent of the

erosion that has shaped Earth’s land surface

• Runoff is all the remaining water that moves over Earth’s surface

• When runoff flows in a thin layer over the land, it may cause a type of erosion called sheet erosion

Runoff and ErosionRills and Gullies• Runoff and the material it

contains move downhill• As it moves, it forms tiny

grooves in the soil called rills

• As the rills flow together, they form gullies

• A gully is a large groove, or channel, in the soil that carries runoff after a rainstorm

Streams and Rivers• Gullies join together to

form larger channels called streams

• A stream is a channel along which water is continually flowing down a slope

• As streams flow together, they form larger bodies of water

• A large stream is a river

Amount of Runoff

Amount of runoff in an area depends on:• The amount of rain an area receives• Vegetation that reduce runoff by absorbing

water and holding soil in place• The type of soil and how they absorb water• The shape of the land and how it is sloped• How people use the land

More runoff means more erosion

What factors aff ect the amount of runoff in a region?

• The amount of rain• The vegetation • Type of soil• How people use the land• Shape of the land

River Systems• A stream grows into a larger stream or river

by receiving water from tributaries• Tributary – a stream that flows into a larger

stream• Drainage basin- the land area from which a

river and its tributaries collect their water• Divide- the high ground between two

drainage basins• Continental Divide

Erosion by Rivers• Through erosion, a river creates V-shaped valleys, waterfalls,

flood plains, meanders, and oxbow lakes• Rivers often form on steep mountain slopes• Waterfalls may occur where a river meets an area of rock that

is very hard• Flood plain- the flat, wide area of land along a river, often

covered when a river overflows its banks• Meander- a looplike bend in the course of a river• As the river widens from side to side, it tends to erode the

outerbank and deposit sediment on the inner bank of a bend• Oxbow lake- a meander that has been cut off from the river;

may form when a river flood

River Formation

Deposits by Rivers

• Deposition creates landforms such as alluvial fans and deltas.

• It can also add soil to a river’s flood plain

• Where a stream flows out of a steep, narrow mountain valley, the stream becomes wider and shallower

• Alluvial fan- a wide, sloping deposit of sediment formed where a stream leaves a mountain range

Deltas

• As a river slows and begins to deposit sediment, it forms a delta

• Delta- Sediment deposited where a river flows into an ocean or lake

• Mississippi River Delta

Soil on Flood Plains

• Deposition occurs during flood

• Deposition of new soil over a flood plain is what makes a river valley fertile

Groundwater Erosion and Depositi on

• Groundwater- water that fills the openings in the soil and trickles into the cracks and spaces in layers of rock

• Groundwater can cause erosion through chemical weathering

• Groundwater combines with CO2 to form carbonic acid which hollows out pockets in limestone and develops large underground caverns or caves

• Stalactite- a deposit that hangs like an icicle from the roof of a cave

• Stalagmite- slow dripping builds up on a cone-shaped deposit on the cave floor

• Karst topography- the collapsing of the roof of a cave due to erosion forming a sink hole

Caves

Review• What is the major cause of erosion on Earth’s surface?

Moving water• Describe the five features formed by rivers and streams as they

erode the land.Valleys, waterfalls, flood plains, meanders, and oxbow lakes

• What are the results of deposition along the course of a stream or river?Deposition results in the creation of alluvial fans and deltas, as well as new soil over flood plains

• How can groundwater contribute to erosion?Groundwater can cause erosion through chemical weathering as acid in the water breaks down limestone, which is carries away by the water in solution

• How is an alluvial fan similar to a delta? How is it different?An alluvial fan and a delta are both deposits of sediment that occur when a stream slows down. An alluvial fan is a wide, sloping deposit formed where a stream leaves a mountain range, while a delta is a deposit when a stream flows into an ocean or lake