RIVER SYSTEMS AND LANDFORMS (2)

RIVER EROSION, DEPOSITION

AND LANDFORMS

1. Introduction

2. River energy

3. River erosion and deposition

4. Landforms: upper, middle and lower reaches

RIVER ENERGY

2. Potential energy: m g h

Mass Acceleration due to gravity

Height

1. Kinetic energy: ½ m v2

Mass Velocity

HEIGHT AND BASE LEVEL

h

Sea level Base level

Lakes and reservoirs can form temporary

intermediate base levels. Source: Christopherson, 2012, p. 401.

BASE LEVEL CHANGES

Sea

Sea

Ice

Water removed from oceans

and stored on land as ice during

glacial periods (‘ice ages’).

Eustatic sea level changes: altering the amount of water in

the oceans.

Present-day coastlines. Source: National Geophysical Data Center.

Coastlines at 18,000 BP. Source: National Geophysical Data Center.

BASE LEVEL CHANGES

Sea

Ice

Isostatic sea level changes: altering the height of the land

relative to the oceans. (1) isostatic depression.

Weight of ice

depresses crust

Sea

Ice

Sea

Ice

Crust depressed into mantle

Land lowered

relative to sea level

Sea

BASE LEVEL CHANGES

Isostatic sea level changes: altering the height of the land

relative to the oceans. (2) isostatic uplift.

Ice melts rapidly; crust rebounds

Sea

Sea level drops

as land rises

North American isostatic

uplift since 6000 BP. Source:

Steven Dutch, University of Wisconsin.

Relationship between river velocity and processes.

Source: Michael J. Pidwirny, Geography Dept., Okanagan University College; after Hjulstrom.

Diameter of sediment (mm)

Flow

velocity

(mm.s-1)

River velocity indicates river energy.

River energy may be used to erode the bed

and banks, and transport eroded material.

PROCESSES OF RIVER EROSION

1. Hydraulic action

2. Abrasion

3. Solution

A turbulent

mountain stream

showing the

effects of

hydraulic action.

Source: Christopherson, 2009, p. 442.

River bed

River surface

TRACTION SALTATION

SUSPENSION

TOTAL RIVER LOAD = BED LOAD

+ SUSPENDED LOAD

+ DISSOLVED LOAD

Processes of river transport. After Christopherson, 2012, p. 409.

Braiding occurs when the sediment load exceeds the

river’s capacity to transport it. Source: Christopherson, 2012, p. 410.

Typical concave longitudinal river profile.

Source: Christopherson, 2009, p. 446.

Upper reach Middle reach Lower reach

A turbulent

mountain stream

showing the

effects of

hydraulic action.

Source: Christopherson, 2009, p. 442.

Cutting of V-shaped valleys in upper reaches.

Source: Steven Dutch, University of Wisconsin.

Low sediment load means river energy can be used mainly for erosion.

Waterfall in New Hampshire. Source: Christopherson, 2009, p. 448.

Waterfalls: common features of the upper reach.

Source: Christopherson, 2012, p. 414.

Waterfalls: common features of the upper reach.

Source: Christopherson, 2012, p. 414.

Headward erosion of Niagara Falls.

Source: Christopherson, 2012, p. 415.

Niagara Falls. Source: Christopherson, 2012, p. 415.

Niagara Falls. Source: Christopherson, 2012, p. 415.

Niagara Falls. Source: Christopherson, 2012, p. 415.

The Mississippi River.

Source: National Park Service/Mississippi National River and Recreation Area.

Cut bank

(steep)

Point bar

(gentle)

Cut bank

(steep)

Characteristics of a meander bend.

Meandering streams: common features of the

middle reach. Source: Christopherson, 2012, p.410.

Point bar development on a meander on the

Manu River, Peru. Source: Erin McManus, Wake Forest University.

Formation of ox-bow lakes from meander bend.

Source: Christopherson, 2012, p.411.

Ox-bow lake

Itkillik River, Alaska. Source: Christopherson, 2012, p.411.

Formation of ox-bow lakes from meander bend.

Source: Christopherson, 2012, p.411.

The Mississippi River.

Source: National Park Service/Mississippi National River and Recreation Area.

Meanders in the Mississippi River. Source: Christopherson, 2012, p.416.

Erosion

Deposition

Floodplain formation and features.

Floodplain formation and features.

Source: Christopherson, 2012, p. 417.

Alluvial terraces, formed from an increase in

potential energy due to a change in base level.

Source: Christopherson, 2012, p. 418.

Entrenched meanders formed from rejuvenation.

Source: Christopherson, 2003, p. 444. See also Christopherson, 2012, p. 413.

Entrenched meanders formed from rejuvenation.

Source: Christopherson, 2012, p. 413.

The Nile delta:

an arcuate delta.

Aswan High Dam

The mouths of the

Ganges entering

the Bay of Bengal.

An arcuate delta.

Source: Christopherson, 2012, p. 419.

The Mississippi delta: a bird’s foot delta.

Satellite image of the Mississippi delta.

Source: Christopherson, 2012, p. 421.

Lake Pontchartrain

New Orleans

Mississippi

River

Atchafalaya

River

3000 BP

1500 BP

1000 BP

700 BP

500 BP

Evolution of the Mississippi delta over 3000 years.

Source: Christopherson, 2012, p. 421.

Location

map of the

Mississippi

delta area.

Source:

Christopherson, 2012,

p. 421.

Source: Times-Picayune

Source: Times-Picayune