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Erosion

• The process by which water, ice, wind or gravity moves fragments of rock and soil.

Water Erosion

• Rivers, streams, and runoff

Fast moving streams and rivers carry big and small rocks downstream.

Slower moving water carries smaller rocks and soil downstream.

Moving water can also cause soil erosion— carrying the soil away to a different location.

Sometimes a side of the hill is washed away by running water. The soil and rocks move down the hill in a landslide.

Ice Erosion

• Glaciers

water deposition

Where a river meets the ocean is called the mouth of the river. Soil carried by a river is deposited at the mouth and new land is formed. This new soil-rich land is known as a delta.

Wind Erosion

deposition

• is the exogenic process that happens after erosion.

• This process adds sediments, soil and rocks to a landform or land mass.

Wind Deposition

Sand dunes are large deposits of sand dropped when the wind stopped blowing. The location of the sand dunes shifts frequently.

Glacial Deposition

When glaciers melt, they drop or deposit the rocks they were carrying.

A moraine is the

rocks and soil left

behind a melting glacier. Glaciers pick up rocks and dirt that travel along with the glacier until it eventually melts and is left behind as till.

Forces that cause erosion

1. Water

a. Rivers and Streams

b. Waves, Beach and Coast

2. Wind

3. Glaciers

4. Ground water

Running Water

• Running water is the most important geologic agent in eroding, transporting and depositing sediment

• Nearly every landscape on Earth shows the results of stream erosion or deposition

Running Water

Stream - a body of running water, confined to a channel, that runs downhill under the influence of gravity

– Headwaters - upper part of stream near its source in the mountains

– Mouth - place where a stream enters sea, lake or larger stream

• Channel-along,narrow depression eroded by a stream into rock or sediment

• Streambanks-sides channel •Streambed – channel bottom

Running Water

Drainage Basins

• Drainage basin - the total area drained by a

stream and its tributaries

– Tributary - a small stream flowing into

a larger one

Drainage Patterns

• Drainage pattern - the arrangement, in map view, of a stream and its tributaries

– Most tributaries join the main stream at an acute angle, forming a V or Y pointing downstream

– Dendritic - drainage pattern resembling the branches of a tree

–

Radial pattern - streams diverge outward like the spokes of a wheel

• Typically Formonconical Mountains (volcanoes)

Drainage Patterns

– Rectangular pattern - tributaries have frequent 90° bends and join other streams at right angles

– Trellis pattern - parallel streams with short tributaries meeting at right angles

Stream Erosion

– Stream velocity controlled by stream gradient (slope), channel shape and channel roughness

• Maximum velocity near center of channel

Stream Erosion

• Channel shape and roughness

– Both affect stream velocity due to

drag

– Narrower, deeper channels allow faster flow

– Smoother channels allow faster flow

– Wider, shallower channels decrease speed

– Rougher channels decrease flow speed

INC. LYCEUM OF ALABANG INC.

• Stream erosion occurs by the following:

– Abrasion - grinding away of stream channel by the friction and impact of the sediment load

– Attrition – collision between sediments breaking into smaller more rounded pebbles

– Hydraulic action - ability of flowing water to pick up and move rock and sediment

– Solution - dissolving of rocks by action of acidic water (e.g., limestone)

Floodplain - flat valley floor composed of sediment deposited by the stream

Before Flooding (August 14, 1991)

After Flooding (November 7, 1993)

Missouri and Mississippi River spill over into floodplain. green = vegetation, red = recently plowed fields (bare soil)

INC. LYCEUM OF ALABANG INC.

• Floodplains Are Broad strips of land built up by sedimentation on either side of a stream channel

– Floodplain Sediments Left behind as flood waters slow and recede at the end of flood events

– Main channel has slightly raised banks with respect to the floodplain known as natural levees

Canyons

Canyons are large valleys created by a river or stream.

This simple animation provides you with a visualization of how the Colorado River has "downcut" into the rock layers of the Grand Canyon.

Canyons demonstrate both weathering—the breaking down of rock into smaller pices and erosion—these pieces of rock being moved to a new location.

• Sediment load transported by a stream can be subdivided into bed load, suspended load, and dissolved load

1. Bed load - large or heavy particles that travel on the streambed

2. Suspended load - sediment that is small/light enough to remain above the stream bottom by turbulent flow for an indefinite period of time

3. Dissolved load - dissolved ions produced by chemical weathering of soluble minerals upstream

Sediment Transportation

Sediment Deposition

• Sediments are temporarily deposited along stream course as bars and floodplain deposits, and at/near its end as deltas or alluvial fans

• Bars - ridges of sediment (usually sand or gravel) deposited in the middle or along the sides of a stream

– Braidedstreamscontainsediment deposited as numerous bars around which water flows in highly interconnected rivulets (heavy sediment shallow, wide channel)

• Delta - body of sediment deposited at the mouth of a river when flow velocity decreases

• Alluvial fan - large, fan- or cone-shaped pile of

sediment that forms where stream velocity decreases as it emerges from a narrow mountain canyon onto a flat plain.

Flooding

• When water levels rise and overtop the banks of a river, flooding occurs

– Natural process on all rivers – Can cause great damage in

heavily populated areas

– High velocity and large volume

of water causes flood erosion

– Slowing of waters as flood ends causes flood deposits (usually of silt or clay-sized particles) to be deposited in the floodplain

Flooding

• Urban flooding

– Paved areas and storm sewers increase

runoff by inhibiting infiltration

– Rapid delivery of water to streams increases peak discharge and hastens occurrence of flood

• Flash floods

– Local, sudden floods of large volume and

short duration

– Typically triggered by heavy thunderstorms

Stream Valley Development

• Incised meanders

– Retain sinuous pattern as they cut

vertically downward

– May be produced by profound base level changes, as when rapid tectonic uplift occurs