As you know from Chapter 2, weathering is the breakdown of rocks into smaller pieces.

So what is erosion?Erosion is the movement of the broken

or weathered sediment

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Gravity – The Driving Force of ErosionThe main force for all erosion!!!

Everything on earth is being pulled toward the earth’s center by gravity.

Gravity pulls all loose particles down along a slope

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Agents of ErosionWATERWATER is the most important agent of

erosionIce, wind, and glaciers are also agents of erosion

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Water-Water- moves far more material than any other agent of erosion

• Streams– Sediments are rounded

Small pebbles, rocks, and sand wear down bedrock through the process of abrasion. This rounds off the pebbles and sand grains.

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STREAMS CARRY MATERIAL IN … the LOAD

• Solution: dissolved material (includes salts)

• Suspension: particles carried in the flow of the water (Clay, silt, sand)

• Bed Load: particles carried by rolling, bouncing, or dragging along the stream bed (Pebbles, cobbles, boulders)

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http://www.classzone.com/books/earth_science/terc/content/visualizations/es1303/es1303page01.cfm?chapter_no=visualization

Water Velocity determines the size particle that can be moved via suspension and bed load

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Characteristics of Streams

Water always flows ___________downhilldownhill

Moving water in a stream has friction with the bed of the stream.There is also friction between the surface water of the stream with the air.

As Velocity of the stream Friction

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Two factors that Two factors that determine stream determine stream

velocityvelocityVe

locity

Velocity

Volume Slope

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Effect of Discharge

When volume of flow _______ the average stream velocity ______

This is because as the stream’s discharge increases, the stream becomes deeper and wider in order to move the greater amount of water being supplied to it.

As a result, less water is in contact with the frictional stream bed.

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Stream Velocity in a Straight ChannelFastest in the middle, just below the surface

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Erosion by Streams• The work of a stream – the higher

the elevation the greater the potential energy of a stream.

500 meters 500 meters

50 m

eter

s

100

met

ers

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Ability to Transport Sediments • Velocity determines the size of

sediments that the stream can move.• The faster the velocity, the larger the

sediments that can be transported.Which river can carry larger sediment? Such as

cobbles and boulders?

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• A stream has a velocity of 200 cm/sec.

• What size sediments can this stream transport?

Pebbles, sand, silt and clay

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This stream has a velocity of 300 cm/sec.

What size sediments can this stream transport?

Clay, silt, sand, pebbles, and cobbles

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• The largest particle diameter this river can transport is 1.0 cm.

• What is the river’s approximate velocity?

100 cm/sec.17

Drainage Basins• All of the land

that drains into a particular river or stream is called its drainage basin or watershed.

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Drainage Basins• Mississippi River Drainage Basin- There is no

stream inside the shaded area that doesn't eventually flow into the Mississippi.

• Every single drop of water that flows from the Mississippi River out into the Gulf of Mexico fell as precipitation within that drainage basin (shaded area)

High elevation?

Low elevation?

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Basin

Divide Divide

The higher land that separates drainage basins is called a divide. 20

Now let’s look at some specific watersheds.

• You will need colored pencils tomorrow.

• We will pick this up again, in a few days, with deposition.

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• Deposition: the dropping of transported materials (sediments) in new locations

• Also known as sedimentation

• There is no deposition without erosion.

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• the larger the particle, the faster it will settle (all other things being equal)

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• What role does particle size have on the deposition rate?

Reasons for this observed relationshipTi

me

Particle Size

The weight of the particle is greater than

the upward force of friction

0 se

cond

s60

sec

onds

Large

(Boulders)

Small

(clays)

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What happens to the velocity of a stream when it flows into a large

body of water?

Sedimentation at the Mouth of a River

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Deposition (sedimentation) at the Mouth of a River

• When a river enters the sea or any other large body of water its velocity suddenly drops.

• This causes deposition to begin.

• Because the current doesn't stop completely at the mouth of a stream horizontal sorting occurs.

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The largest, roundest, and most dense particles are deposited closest to the shoreline.

As you move out from the shoreline the pattern will show a gradual change from coarse to fine, from roundest to flattest, and from most to least dense.

Ions in the sea water may come together if the solution becomes saturated and precipitate out of the solution.

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What is the effect of stream velocity on the patterns of deposition?

When a stream has a high velocity _____________occurs.erosion

When the velocity of a stream decreases _____________ occurs.deposition

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The velocity changes in the S-shaped curves called MEANDERS.

Fastest on the outside of the curve =Fastest on the outside of the curve = erosionerosionSlowest on the inside of the curve =Slowest on the inside of the curve = depositiondeposition

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Water velocity is greatest along the outside curve of a meander (cut bank). Erosion is dominant.

Water velocity is slowest along the inside of a meander (point bar). Deposition is dominant.

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A.

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B.

F.

D.

C.

E.

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Oxbow lakes are associated with OLD STREAMSOLD STREAMS

oxbow lakes

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http://www.youtube.com/watch?v=ReCt2AcydCg&feature=related

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The life cycle of streams

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Youthful, mature, or old?

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Flood Plains• The part of a valley where rivers flood

is called the flood plain.

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Deltas• Fan-shaped

sediments deposits at the mouths of rivers are called deltas. The Nile, Amazon, and Mississippi rivers have well-developed deltas.

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• Alluvial fans form as the result of sediments being washed down the

side of a mountain.

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You are almost ready…

• We have a little more work to do before we do the stream tables.

• Please bring devices tomorrow, we are going to download a new app for use with the stream tables…it will be awesome

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