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 TransportLowlands, 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) DrainageStreams 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,

India-Nepal-Tibet

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