Erosion and Landscape Evolution

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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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

The river only erodes a slot. Other processes carry material from the valley walls to teh river to be carried awar. So rivers only deepen their valleys; other processes widen them.


 

Constructive and Destructive Processes

Highlands

Transport

Lowlands, Coastal Plain, Lakes and Seas


The Ideal Stream Cycle (W.M. Davis, 1880)

Not a Literal Time Sequence

Youth

Youthful humid-climate landscape

Maturity (Early)

Early mature humid-climate landscape

Maturity (Late)

Late mature humid climate landscape

Old Age

Old age humid climate landscape;

About River Meanders

Rejuvenation

rejuvenation of old-age landscape

Rejuvenation of an old-age landscape.

Rejuvenation of an early mature landscape

Rejuvenation of an early mature landscape.

Why the Stream Cycle Doesn't Explain Everything


Superposed (Antecedent) Drainage

Streams Cut Right Through High Topography

Antecedent Drainage

  1. Rejuvenation of Streams Flowing on a Peneplain
  2. Valley Cut Downward Through Overlying Deposits
  3. Uplift While Stream Cuts Downward

 Rejuvenated Peneplain: the Northeastern US

peneplain of northeast U.S.

Harrisburg, PA region

Drainage Diversion


Arid and Humid Weathering Compared

  Humid Climates Arid Climates
Rain Frequent Rare, May Be Seasonal, Often Violent
Soil Cover Thick Thin or Absent
Vegetation Thick Sparse-no Continuous Cover
Chemical Weathering Intense Weak
Overall Landscape Evolution Mostly Uniform Processes Episodic Processes

Arid Erosion Cycle

Youth

Young arid landscape

Maturity

Late mature arid landscape

Some geologists think this happens in all landscapes but is modified by soil creep and mass wasting in humid climates.

Old Age

Old age arid landscape


Deltas

evolution of Mississippi delta Changes of the Mississippi River delta since 3000 B.C. Note that there are intervals of overlap for some deltas. It seems obvious the Mississippi is long overdue for a shift to the west, and in fact a huge spillway complex allows flood water to flow down the Atchafalaya, where a substantial delta has already begun to form. We don't simply let the river change its course because it would isolate New Orleans and Baton Rouge economically, it might possibly isolate the entire river if the new channel can't be navigated, and it would disrupt the wetland ecosystem along the Atchafalaya. Sooner or later, however, it will happen. Lake Ponchartrain (the big lake north of New Orleans) is actually a former bay cut off by one of the Mississippi's delta lobes.

Where are the earlier deltas? Pre-Pleisocene deltas fill in most of Louisiana but are mostly buried and not known in detail. Pleistocene deltas formed when sea level was lower than at present and are somewhere under the present Gulf of Mexico. About 7,000 years ago sea level began to stabilize following the melting of the Pleistocene glaciers. That time coincides with the onset in many places of intensive settled agriculture and early civilization. Not only did it become possible to farm river deltas without having to resettle frequently as sea level rose, but the infilling of river valleys slowed, making it possible to farm flood plains as well.

delta of Colorado river The Imperial Valley of California is below sea level because it is a former part of the Gulf of California cut off by the Colorado River delta. The valley has a year-round growing season and is one of the most important agricultural areas in the U.S. At one time it was almost entirely dry, but about 1910 an irrigation canal from the Colorado River overflowed and flooded a large part of the center of the valley. By the time the flood was stopped, residents decided that a lake might not be a bad idea, so it is still there. It's called the Salton Sea and is maintained artificially.
Okavango Delta, Africa Niger delta, Africa
Some deltas can actually be inland. The Okavango River of Africa flows into a closed basin, where it soaks into the ground or evaporates. The Okavango Delta is one of the world's great wildlife habitats. Other inland deltas include the Niger where it first enters the Sahara Desert, and the Sacramento-San Joaquin delta in California.

Waterfalls

Resistant Ledge

Hanging Valley


Lakes

Limited Lifetime Thousands - Millions of Yr.

How They Form:

Grabens

Scour

Damming

Small Lakes

How Lakes Die


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Created 12 Jan 1997; Last Updated 27 January 2012

Not an official UW-Green Bay site