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

meanders floodplain

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!