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
1. Kinetic energy: ½ m v2
Mass
Velocity
2. Potential energy: m g h
Mass
Height
Acceleration due to gravity
HEIGHT AND BASE LEVEL
h
Base level
Sea level
Lakes and reservoirs can form temporary
intermediate base levels. Source: Christopherson, 2012, p. 401.
BASE LEVEL CHANGES
Eustatic sea level changes: altering the amount of water in
the oceans.
Sea
Ice
Water removed from oceans
and stored on land as ice during
glacial periods (‘ice ages’).
Sea
Present-day coastlines. Source: National Geophysical Data Center.
Coastlines at 18,000 BP. Source: National Geophysical Data Center.
BASE LEVEL CHANGES
Isostatic sea level changes: altering the height of the land
relative to the oceans. (1) isostatic depression.
Ice
Weight of ice
depresses crust
Sea
Ice
Ice
Land lowered
relative to sea level
Sea
Sea
Crust depressed into mantle
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
Sea
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.
Flow
velocity
(mm.s-1)
Diameter of sediment (mm)
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.
Processes of river transport.
After Christopherson, 2012, p. 409.
River surface
SUSPENSION
TRACTION
SALTATION
River bed
TOTAL RIVER LOAD =
BED LOAD
+ SUSPENDED LOAD
+ DISSOLVED LOAD
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.
Characteristics of a meander bend.
Cut bank
(steep)
Point bar
(gentle)
Cut bank
(steep)
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.
Floodplain formation and features.
Deposition
Erosion
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.
Lake Pontchartrain
Mississippi
River
Atchafalaya
River
Satellite image of the Mississippi delta.
Source: Christopherson, 2012, p. 421.
New Orleans
Evolution of the Mississippi delta over 3000 years.
Source: Christopherson, 2012, p. 421.
1500 BP
3000 BP
700 BP
1000 BP
500 BP
Location
map of the
Mississippi
delta area.
Source:
Christopherson, 2012,
p. 421.
Source: Times-Picayune
Source: Times-Picayune