Erosion and Landscape Evolution
How Do We Know Rivers Cut Their
Valleys?
John Playfair, 1800
• Tributary valleys almost always join the main
valley at exactly the same elevation, even
though the valleys may begin many miles
apart. This is very unlikely unless the rivers
have cut the valleys.
• How Rivers Widen Valleys
Constructive and Destructive
Processes
Highlands
•
•
•
•
Erosion Dominates
Destructive Processes
History not Preserved
Little Geological Record
Transport
Lowlands, Coastal Plain, Lakes and Seas
•
•
•
•
Deposition Dominates
Constructive Processes
History Preserved
Good Geological Record
Stream Abrasion, Marathon County
Stream Potholes, Marathon County
Mega-Potholes, St. Croix Valley
Anatomy
of a
Drainage
System
The
Continental
Divide,
Colorado
Stream Order
The River That Did This….
Looks Like This Near Its Source
The Ideal Stream Cycle
(W.M. Davis, 1880)
Not a Literal Time Sequence
• Youth
• Maturity
• Old Age
• Rejuvenation
Youth
•
•
•
•
•
V-Shaped Valley
Rapids
Waterfalls
No Flood Plain
Drainage Divides
Broad and Flat,
Undissected by
Erosion
• Valley Being
Deepened
• General Agreement
on this stage, lots
of examples
Youthful Landscape, Arizona
Maturity (Early)
• V-Shaped Valley
• Beginnings of Flood
Plain
• Sand and Gravel Bars
• Sharp Divides
• Relief Reaches
Maximum
• Valleys stop
deepening
• General Agreement
on this stage, lots of
examples
Young-Mature Landscape, California
Mature Landscape, Kentucky
Maturity
(Late)
• Valley has flat
bottom
• Narrow Flood Plain
• Divides begin to
round off
• Relief diminishes
• Sediment builds up,
flood plain widens
• River begins to
meander
• Many geologists
believe slopes stay
steep but simply
retreat.
Old Mature Landscape, Tennessee
Old Age
• Land worn to
nearly flat surface
(peneplain)
• Resistant rocks
remain as
erosional remnants
(monadnocks)
• Rivers meander
across extremely
wide, flat flood
plains
Monadnock, Colorado
Monadnocks, Maine
Old Age Landscape, South America
The Onset of Old Age? Indiana
Old Age? Or Maybe Not: Nebraska
Old Age? No! (Wisconsin)
Rejuvenation
• Some change causes stream to speed up
and cut deeper.
– Uplift of Land
– Lowering of Sea Level
– Greater stream flow
• Stream valley takes on youthful
characteristics but retains features of older
stages as well.
• Can happen at any point in the cycle.
Rejuvenation, Utah
Rejuvenation of an old-age landscape
Rejuvenation, San Juan River, Utah
Rejuvenation of an early mature
landscape
Machu Pichu, Peru
Machu Pichu, Peru
Why the Stream Cycle Doesn't Explain
Everything
• Rises and falls in sea level during the ice ages
rejuvenated most landscapes to some extent.
• Climate changes mean that mass-wasting processes
in temperate regions may have undergone radical
changes repeatedly in the last few million years.
• In places where conditions have remained uniform
for long times, like the stable interiors of Africa,
Australia and South America, the ideal stream cycle
seems to work best.
Sea Level and River Profile
Superposed (Antecedent) Drainage
Streams Cut Right Through High Topography
Rejuvenated
Peneplain:
the
Northeastern
US
Rejuvenated
Peneplain
Superposed
Drainage,
Delaware
Water Gap
Water Gap, Pennsylvania
Cumberland Mountains, Virginia
Cumberland Gap
Devil’s Gap, Wyoming
Approach to Devil’s Gap
Rivers and
Crustal
Movement,
California
Tectonic Uplift, Colorado
Tectonic Uplift, Grand Canyon
The
Ultimate
Antecedent
Drainage,
IndiaNepalTibet
Drainage Diversion
The Huang
He: “China’s
Sorrow”
• 1887: 2,000,000 dead
• 1931: 3,700,000 dead
• 1938: The Chinese
dynamite levees to
slow the Japanese; half
a million Chinese died.
River Diversions in the Caspian Region
Stream
Piracy:
Northeast
England
Why is the Danube Blue?
Piracy on the Danube
Flood, Ecuador
Flood, Green Bay, June 1990
Flood, Green Bay, June 1990
Building Smart in a Flood Plain
Channeled Scablands, Washington
Fluid Flow is Scale-Invariant
Erosion of Bedrock River Beds
Scabland Terrain, Oregon
Erosion of
Soft River
Beds
Mega-Gravel Bar, Washington
Mega-Flood
Deposits,
Washington