1. RIVERS

2. When they are young, rivers ideally start out relatively straight in map view, entrenched in V-shaped valleys. You’ll also find plenty of waterfalls and rapids at this “Youth” stage.
As time goes by, the river loses the gravitational impetus to incise any deeper. The river now begins to meander side to side, and as it does so, enlarges the size of its valley by lateral erosion. It is “Mature.” As time goes by, the valley walls get further and further apart.

3. If enough time goes by, the river can enlarge the size of its valley so much that you can’t really tell it’s a valley any more.
At this stage, meandering can get pronounced enough to fold back on itself and create oxbow lakes (visible in the map view of the “Old Age” stage).
The story could end here. However, if base level were to drop anew, the river will begin to incise again, producing a valley profile (cross-section) that looks pretty much identical to the “Youth” stage. It has been made young again, or “Rejuvenated.” In map view, however, you can see from the meandering shape of the re-incised valley that the river must once have been at the “Old Age” stage.

6. Outline
Here, we will learn on these processes:
- River Erosion
- River Transportation
- River Deposition
River Erosion
River Erosion is the river erodes away its channel vertically and laterally.
Vertical erosion is the downward erosion which deepens the river channel.
Lateral erosion is sideward erosion which widens the river channel.
The work of erosion depends on the energy of the river.

7. River Transportation
A river uses its energy to carry or transport eroded materials such as mud, sand, boulders and dissolved materials. These materials are called its load.
River transport their load by 4 processes:
Traction (rodadura)
Saltation
Suspension
Solution

8. River Landforms
Potholes
Rapids
Waterfalls
Gorges
Potholes (marmitas de gigante) are holes in the river bed.
They vary in size from a few centimetres to several metres in diameter.
Pothole formation:
Pebbles carried by the river are swirled around on the river bed. This action erodes the rock on the river bed forming potholes.
Over time, they may widen and join with other potholes to form larger potholes, and the whole river bed is deepened.

9. Rapids
Rapids are stretches of fast-flowing water tumbling over a rocky-shallow riverbed.
Different resistance among rocks cause rapids.
They are formed when the water goes from one hard rock that resists the water’s erosion to a softer rock that is easier eroded.
As result, the rapids form a series of steps.

10. Waterfalls
A waterfall is formed where there is the sudden fall in height. The river plunges and hits the bottom of the river bed with great force.
Rocks are swirled around to form a deep pool at the river bed known as a plunge pool.

11. Gorges
Gorges are formed when rivers erode vertically through resistant rocks.
Since these rocks are not easily eroded, the sides of the valley remain steep.
Gorges is a deep valley, narrow and steep sided.

12. River Deposition
There are many types of River Deposition:
Floodplain
Meanders
Delta
Ox-Bow Lake
Floodplain
A floodplain is a low-lying plain on both sides of a river that has repeatedly overflowed its banks and flooded the surrounding areas.
When the floods subside, alluvium is deposited on the floodplain.
The larger materials, being heavier, are deposited at the river banks while the finer materials are carried and deposited further away from the river.
The larger materials at the river banks build up into embankment called levees.

13. Meanders
Meanders are loop-like bends in a river.
The erosion takes place on the outer bank and deposition on the inner bank.
Gradually, a steep river cliff is formed on the outer bank, making it concave in shape.
On the inner bank, deposition of alluvial materials produces a gentle slip-off slope and the bank takes on a convex shape.

14. Ox-Bow Lake
An oxbow lake is a crescent-shaped lake formed on a river when a meander has been cut through and abandoned.
When a river meanders in very big loops, the outer bank is so rapidly eroded that the river cuts through the narrow neck of the meander.
The river then flows straight through the channel. When deposition seals off the cut-off from the river channel, an oxbow lake is formed. It may silt up and eventually dry up.
1) The river is starting to meander. Erosion is greater on the outside of the bend, deposition more on the inside.
2) Large meanders have formed
3) The river cuts through the meander, leaving a straighter section and an ox-bow lake.

15. Delta
A delta is a flat piece of land built-up from layers of sediment deposited by a river where it enters a lake or calm sea.
The river may have to branch into smaller distributaries to carry the water to the sea.

17. The Ideal Stream Cycle (Davis, 1880)
Youth
Maturity
Old Age
Rejuvenation
Youth
V-Shaped Valley
Rapids
Waterfalls
No Flood Plain
Valley Being Deepened

18. How Do We Know Rivers Cut Their Valleys? How Rivers Widen Valleys?
The river only erodes a slot. Other processes carry material from the valley walls to the river to be carried away. So rivers only deepen their valleys; other processes widen them.

19. 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

20. 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)

21. 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.

23. Evolution of the River Valley
River valleys can also be categorized into maturity stages based upon their rate of water flow as well as the formation of certain features within the valley.
Youthful river valleys typically have fast flowing rivers and steep valley walls which rapidly erode.
When a river valley reaches maturity, water velocity slows and the river broadens out to meander from side to side within the valley.
Finally in old age the river valley has broadened out to form wide flood plains with extensive meandering. Features such as secondary flow channels and oxbow lakes can been seen in these valleys.

24. When the river evolution is interrupted due to uplift, change in sealevel, or climate change, the erosive power of the rivers may increase again, and they begin downcutting through their previously deposited floodplain deposits. They develop a new floodplain at a lower level. The river valley has been terraced.
This process can occur repeatedly, and we can get a series of river terraces (or fluvial terraces) in a river valley.
Terraced river valleys are quite common, due to the strong changes of climate and sealevel in recent Earth history.
Because the underlying processes are global in nature, episodes of river terraces can be used to correlate recent Earth history over large areas and even from continent to continent