Chapter 4: Weathering and Erosion TOPIC 6 : River Deposits ; Flooding.

Chapter 4: Weathering and Erosion

TOPIC 6 : River Deposits ; Flooding

Deposition

• The amount and size of a river’s load depends on discharge, which in turn depends on speed.

• When a river slows down, it loses it’s ability to carry some of it’s load.

• Some of the load gets deposited where the river loses speed. Heaviest load (bedload) is the first to get deposited.

Deposition locations

• The inside bank of meanders (looping bends)– In sharp turns, water on the inside of the turn

slows down while it speeds up on the outside bank. (inside = deposition ; outside = erosion)

• Near and at the mouth of a river– As rivers approach their base level (elevation

of what they are flowing into) they lose most of their gradient and speed.

Depositional Features of Rivers

• POINT BARS and FLOODPLAINS– Form along the inside bank of meanders– Point bars = rocks, gravel, sand– Floodplains = soil (sand, silt, clay)

• Flat areas of fertile soil that form along the banks of a river and are covered by water during flood stage.

• DELTAS and ALLUVIAL FANS– Fan-shaped land formed at the mouth of a

river from deposits of fertile sediment.

CUT BANK

POINT BAR

Cut Bank

CUT BANK

Cut Bank Erosion

meandersfloodplain

Formation of Point Bars and Floodplains

• http://csdms.colorado.edu/wiki/Movie:Floodplain_Evolution

• Stream table meandering

Deltas

• Delta formation in stream table

The Nile Delta (Eqypt)

Benefits and dangers of rivers

• In addition to providing fresh water, river deposits produce very fertile farmland.

• Floodplains and deltas are among the most fertile areas for farming.

• Floodplains and deltas are prone to flooding when rivers overflow their banks (known as “flood stage”). Crops and structures built on these may be destroyed during floods.

Controlling the river

• The risk of flooding can be controlled by the construction of:– Dams – a barrier that redirects the flow of

water to another area; controls how much water flows through the channel.

• Dams can also use the energy of a river to produce mechanical or electrical energy.

– Levees – a barrier built along the banks of a river to prevent overflow during flood stage.

Taming the River- Dams

A dam

Levee

This guy really wants to be left alone!