EROSION-

The transport of weathered materials…

Major Erosive Agents:

Running WaterGLACIERSWINDOCEAN CURRENTS AND

WAVESMASS WASTING (GRAVITY!)

Also known as MASS MOVEMENTAvalanches or rock fallsLand or Mud SlidesSlump or FlowsCreep

EROSION by gravity

1995 Landslide in La Conchita, CA.

The two forces involved in all mass movements are GRAVITY (pulling the material down) and FRICTION (keeping the material in place).

Typically excessive amounts of water lessen the force of friction and lead to a landslide.

New York City’s

Henry Hudson

Parkway in

Upper

Manhattan

A 75 foot

retaining wall

collapsed on

May 12th, 2005.

Running water move more sediment than any other agent of erosion.

What happens to rainfall when it hits earth????

Some evaporates Some is absorbed and becomes ground

water Some is run off

What determines if rainwater becomes runoff or groundwater?

Ground Conditions:Saturated vs. unsaturatedVegetation – amount of plantsSlope or Gradient of the Land

Running Water – Most of North America is above sea level. Water flows high to

low sea level due to gravity

When Runoff becomes confined to a channel – Stream Exists

Watersheds or Drainage Basins

Smaller stream that flows into a larger stream is called a tributary

Tributaries and land that contribute to the stream is called a drainage basin or watershed

(COMPARED TO A BATHTUB. WATER THAT COLLECTS IN A BATHTUB FLOWS TOWARD THE DRAIN.)

The drainage basin of one stream is separated by a drainage divide (ridges or mountains – higher elevations)

HOW WATERSHEDS/DRAINAGE BASINS FORM – RED LINE IS DRAINAGE DIVIDE

Watersheds: Continental Divide and Appalachian Mountains

Water goes from high sea level to low sea level

Watershed Regions of New York State

3 factors affecting Stream Velocity

Gradient – slope of the stream

Discharge – volume of water

Stream channel shape

Stream Velocity in a Straight ChannelFastest – middle just below surface

The velocity changes in the S-shaped curves called MEANDERS. Fastest on the outside of the curve – erosionSlowest on the inside of the curve - deposition

STREAMS CARRY MATERIAL IN … 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)

THE GREATER THE VELOCITY OF A STREAM…

THE LARGER THE SEDIMENT PARTICLES IT CAN CARRY

THE MORE TOTAL SEDIMENTS IT CAN CARRY

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

Stream Development – Early Stage

The landscape is continuously being reshaped by stream erosion. Plateaus can be cut by rivers to form canyons. Valleys can be carved in mountains as rivers flow through them.

As the stream begins to down cut into the land – waterfalls and rapids form

Stream Development – Young Stage

V-Shaped Valley Rapids and Waterfalls No Flood Plain – a level plain that borders

stream

V-SHAPED VALLEY…

RAPIDS AND WATERFALLS

Rapids - Different resistance among rocks cause FAST-FLOWING WATER

Waterfalls – Rapids develop into waterfalls as softer rock erodes

Stream Development - Mature Stage Lateral erosion widens the valley

profile – slope decreases River meanders pronounced A FLOOD PLAIN develops

As the stream grows larger – it begins to shift its course in a series of bends or curves called meanders seeking out the lowest elevation

Point Bars show depositionCut Banks show erosion

Cut bank on outside of curve due to higher velocity of water – more erosion

Cut bank on outside of curve

Point Bar on inside of curve

Stream Development - Old Age

Very Wide Flood Plain Land worn down to flat surface Oxbow Lakes

Stream Development – Old Age

Eventually, it takes too much energy to keep meandering, so rivers will cut off a bend, resulting in an OXBOW lake…

What is the correct sequence in stream development?

Levee Formation

SEDIMENTS ERODED BY RUNNING WATER WILL HAVE THESE CHARACTERISTICS

ROUNDED and SMOOTHSORTED AND LAYERED

SEDIMENTS ERODED BY RUNNING WATER WILL BE … SORTED – all materials are

about the same size

Contour lines showing head of the stream to the mouth of stream with delta

Velocity slows when entering a large body of water

Alluvial fans form when streams come out of mountains onto flat land – velocity slows and deposition occurs

Erosion by Waves and Long shore Current

Where lake or ocean waters meet land, waves and currents act as agents of erosion, creating unique landscape features

Winds at the ocean’s surface cause waves

Longshore Current

Waves are refracted or bent in the shallow water of the shore

Water near the shore is pushed in one direction along the shore – called longshore current

Sediments are carried in a zig-zag pattern in the SAME direction as the longshore current

The sand trapping effect of the jetties to the north at Charleston, South Carolina has deprived Morris Island of needed sand. As a result, the island has eroded to the point that this previously land based lighthouse is now 400 meters out to sea.

Which direction is the long shore current carrying sediment????

Barrier Islands

Wave Action creates sand bars Waves and Longshore current - can

form barrier islands Ex. Fire Island

Wind Erosion

Sediment – sand or smallerArid regions or ocean coastlines

2 types:

1. Deflation – lowers the land

2. Sandblasting and Abrasion

Sandblasting and Abrasion Wind can only lift sand to about 1 meter

high - results in “Mushroom Rock” Frosted or Pitted sediments – rocks with

flat sides

Windward side

Leeward side