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
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\ | | / Other Processes Widen the Valley:
\ | | / Erosion by Tributary Streams
\ | | / Weathering
\ | | / Mass-Wasting
\ | | /
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Stream Cuts Valley Deeper
(Abrasion)
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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
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
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
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
- Lots of Disagreement from here on; some geologists believe slopes stay steep but simply retreat.
Old Age
;- Very Wide Flood Plain
- Land worn down to flat surface (Peneplain)
- Resistant rocks form residual hills (Monadnocks)
- Pronounced River Meanders
- Cut-off Meanders (Ox-bow lakes)
About River Meanders
- Rivers meander because the landscape is flat.
- Some Landscapes are flat because they are extremely ancient.
- Other Landscapes are flat for other reasons, like recent sediment deposition.
- Hence, some " width="566" height="301" old age" rivers are actually very young.
- Land worn to nearly flat surface (peneplain)
- Resistant rocks remain as erosional remnants (monadnocks)
- Rivers meander across extremely wide, flat flood plains
Rejuvenation
- Uplift of Land (Lowering of Sea Level, sometimes greater stream flow) causes stream to speed up and cut deeper.
- Stream valley takes on youthful characteristics but retains features of older stages as well.
- Can happen at any point in the cycle.
Rejuvenation of an old-age landscape.
Rejuvenation of an early mature landscape.
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.
Superposed (Antecedent) Drainage
Streams Cut Right Through High Topography
- Rejuvenation of Streams Flowing on a Peneplain
- Valley Cut Downward Through Overlying Deposits
- Uplift While Stream Cuts Downward
Rejuvenated Peneplain: the Northeastern US
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
- V-shaped Valleys
- Divides Flat, Undissected
- Much Like Youth in Humid Climates
Maturity
- Slopes Stay Steep, Retreat
- Alluvial Fans
- Playa Lakes
- Pediment
Some geologists think this happens in all landscapes but is modified by soil creep and mass wasting in humid climates.
Old Age
- Inselbergs
- Playa Lakes
- Pediplain
Deltas
 | 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 willhappen. 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.
 | 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. |
 |  |
| 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
- Niagara
In the diagram at left, the percents show river flow in each channel when there is more than one. In about 1000 years the falls will pass Goat Island, the American Falls will be abandoned, and there will only be a single fall. In about 6000 years the falls will retreat to the tip of Grand Island and there will be a triple falls. As the falls retreat, the hard rock of the lip of the falls will decrease in elevation and the thickness of soft rock above will increase. By about 15,000 years (blue) the present falls will be much lower and their retreat will slow to near zero. The river will cut a gorge upstream from the falls in the soft rock, and when the gorge reaches Buffalo in around 30,000 years, it will encounter another layer of resistant rock and a second falls will form.Source: Philbrick, S. S., 1974; What Future for Niagara Falls? Geological Society of America Bulletin. Vol. 85; no. 1, p. 91-98. - Victoria
Hanging Valley
- Glacial: Yosemite
- Non-glacial: Angel
Lakes
Limited Lifetime Thousands - Millions of Yr.
How They Form:
Grabens
- Tahoe 1600'
- Baikal 5600'
- Tanganyika 4000'
Scour
- Great Lakes to 1300'
- Great Slave L. 2000'
- Lake Winnipeg
Damming
- Tectonic - Lake Kioga
Lake Kioga in Uganda is a particularly interesting case of a tectonically dammed lake. The block east of the East African Rift tilted eastward. Prior to tilting the rivers probably joined the Congo and flowed west to the Atlantic. One watershed was tilted so far that the course of the river was cut off and the river filled its own former valleys. The lake now drains out through a former tributary valley and carries the upper Nile. Different colors denote different watersheds. National boundaries are in red and the horizontal black line is the equator. The largest lake is Lake Victoria; the lake north of it is Lake Kioga. See the map below for a detailed view. 
A detailed map of Lake Kioga. Lakes are in blue and marshes in green. The Nile is in purple with red arrows showing flow directions. - Landslide - Spirit Lake
- Lava Flow - Clear Lake, California
Clear Lake is the largest and most misnamed natural lake entirely within California (Lake Tahoe is shared with Nevada). To the west is a steep and short drop to the Russian River, but the former valley there was dammed about a million years ago by lava flows. The valley flooded and found an outlet to the east, where more volcanic rocks slow deepening and erosion of the outlet. - Glaciers - Lake Agazziz
- Delta - Lake Ponchartrain
As already noted earlier, Lake Ponchartrain (the big lake north of New Orleans) is actually a former bay cut off by one of the Mississippi's delta lobes.
Small Lakes
- Volcanic Collapse - Crater Lake
- Sinkholes
- Kettle Ponds
How Lakes Die
- Eutrophication
- Infilling - Only Way to Destroy Very Deep Lakes
- Drainage at Outlet
- Climate Change
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