ErosionErosion

Or. . . How to make molehills out Or. . . How to make molehills out of mountains.of mountains.

What is Erosion?What is Erosion?

• Weathered rock particles (sediments) are transported through the process of erosion.

• Any process that moves sediments from one place to another is called erosion.

• Over time, erosion shapes and lowers Earth’s surface.

Transporting Systems of ErosionTransporting Systems of Erosion

Several components:

• An agent of erosion such as: moving water, wind, ice, or human activity.

• The sediments being moved.

• A driving force.

THETHE DRIVING FORCE DRIVING FORCE

• GRAVITY– Usually works together with water, wind or ice

to move sediments.– Can work on its own.

Mass Movements (Wasting)Mass Movements (Wasting)

Slow mass wasting• Soil creep

Rapid mass wasting• Rockfalls

• Landslides

• Mudflow

• Slump

Running WaterRunning Water

• Most common agent of erosion– Runoff

• Temporary – can carve out rills or gullies

– Streams and rivers• Permanent – confined to a channel

Streams and RiversStreams and Rivers

• Carry sediments in different ways– Solution – dissolved minerals– Suspension – silts and clays– Bedload – largest sediments slide, roll, or bounce

along bottom

Deposition and SortingDeposition and Sorting• A stream looses velocity when entering quiet

waters so its ability to transport sediment decreases. Particles drop to the bottom. This is deposition.

• Large particles settle first. Other particles settle according to size. Smallest particles (clay) drift the farthest and settle last. This is sorting.

Boulders

Cobbles

PebblesSand Silt Clay

Rounded Sediments from AbrasionRounded Sediments from Abrasion

LandformsLandforms

• YOUNG rivers create V-shaped valleys– Most energy

expended in downcutting

– Valley slope quite steep

– River gradient can also be quite steep

• MATURE rivers broaden their valleys.– More energy expended

sideways– Valley slopes less steep– River gradient not as

steep

• Rivers in OLD AGE have very broad valleys– Most energy

expended sideways– Has many meanders– River gradient almost

flat

Erosion can create spectacular scenery.Erosion can create spectacular scenery.Crustal uplift and river erosion created the Grand Canyon over millions of years.

Watershed or Drainage AreaWatershed or Drainage Area

• Often, rivers are fed by many tributaries over a large area. This is called a watershed.

• The largest in the U.S. is the Mississippi River watershed. It drains 42% of the lower 48 states.

The lower Mississippi River has a very broad valley with many meanders. Here, it is a river in old age.

• Imagine trying to pilot a river boat through all of the meanders.

Oxbow LakesOxbow Lakes

• Meanders are often cut off when a river floods. When this happens an oxbow lake is created.

Where Erosion and Deposition Where Erosion and Deposition OccurOccur

Flood PlainFlood Plain

• Rivers with broad valleys are prone to flooding.

• During flooding a river overflows its banks into the wider valley and deposits sediments forming a flood plain.

• Farmers often locate on flood plains because they have fertile soil for growing crops.

• During times of severe flooding there is much property damage. Sometimes at a cost of many billions of dollars.

DeltaDelta

• Where a river enters a quiet body of water its transported sediments deposit at the mouth. This is called a delta.

River areas are diverse ecosystems that are River areas are diverse ecosystems that are inhabited by many species of wildlife.inhabited by many species of wildlife.