Streams. Drainage systems. Drainage basin / watershed. Erosion, overland flow. Hydraulic action, abrasion, corrosion. EROSION. How quickly erosion occurs depends on three things. 1. Flow speed. 2. Turbulencel. 3. Resistance of bedrock. TRANSPORTATION. Stream load. Competence. Capacity. DEPOSITION. Alluvium. Stream flow patterns. Ephemeral streams. Equilibrium. Valley deepening - knickpoint migration. Valley widening and flattening - meanders, floodplains, aggradation. Valley lengthening - headward erosion, delta formation. Stream rejuvenation, degradation, entrenched meanders.

1. Chapter 11
Fluvial Geomorphology

2. Streams
The work that streams do:
• Erosion/Denudation
• Transportation
• Deposition

3. Drainage Systems
• Drainage system—A
branched, hierarchical
network of streams and
tributaries
• Valley—Where a drainage
system is clearly established
• Interfluve—High ground that
separates valleys
(“inter”=between, “fluvia”=
rivers)
• Drainage divide—The
invisible line separating two
drainage basins

4. Drainage basin / Watershed
A single network system; includes both the channeled
valley and any other land surface contributing overland
flow or groundwater to the stream

5. Erosion
•Overland flow—
unchannelized flow of water
– Splash erosion
– Sheet erosion
•Hydraulic action
•Abrasion
•Corrosion

6. Three Processes of Stream Erosion
•Hydraulic action—The physical force of water
pounding on rocks and land materials,
breaking them apart
•Abrasion—Rock materials hitting the bed
(bottom) of the river and its banks (sides)
•Corrosion—The chemical action of water
dissolving minerals and rock material

7. How Quickly Erosion Occurs
Depends On…
1. Flow Speed
2. Turbulence
3. Resistance of the bedrock

8. 1. Flow Speed
Flow Speed
–The faster the water, the more force it has
–Faster water = more erosion

9. Factors influencing flow speed
1. Steeper gradient
– Gradient—The rate of fall in elevation of the stream
surface in the downstream direction (as in 20 ft./mi.)
• Steeper gradient = faster flow = more erosion
2. Volume of flow (discharge)
– More water = higher speed = more erosion
• Flood events move bigger material—and more of it!
3. Channel width—the narrower the channel, the
swifter the flow, for the same volume of water
(Remember the Venturi effect?)

10. How much erosive force?

11. How much erosive force?

12. How much erosive force?

13. 2. Turbulence
•Water swirling and tossing, not smooth flow
•Turbulence is determined by:
–Flow speed
•Faster flow = increased turbulence
–Roughness of the stream channel
•A rough, irregular channel = more turbulence

14. 3. Resistance of bedrock
Harder rocks = less erosion

15. Transportation: Stream Load
Stream load (rock material transported by streams) is
carried in three forms:
1. Dissolved load—dissolved minerals carried in solution
2. Suspended load—small particles that never touch the
stream bed (Most of the stream load is suspended load)
3. Bedload—larger rock fragments that drag, roll, skip or
bounce along the stream bed
Material is picked up,
dropped and picked
up again

16. Transportation: Stream Load

17. Transportation: Competence
• The faster the stream is flowing, the larger
the particles it can transport
• This measurement is called competence
– Competence varies to the sixth power of the
water’s speed
If flow speed doubles:
26 = 64 times the size!
– This is why flood events are so significant in
stream transportation

18. Transportation: Capacity
•Capacity—A measure of the amount of solid
material potentially transported (volume/time
past a given point: gal/sec)
•The capacity of a given stream depends on:
1. Water volume
(volume↑, capacity↑)
2. Flow speed
(flow speed↑, capacity↑)
3. The type of load material
(lighter material, capacity↑)

19. Deposition
•Deposition occurs when either flow speed or
volume decrease
•Conditions that cause deposition:
–Change in gradient
–Channel widening
–Change of direction
–Flowing into less active water

20. Deposition: Alluvium
•Alluvium—Any stream-deposited debris
–Smaller particles carried farther than large ones
•Sorting—Based on size. As stream flow decreases,
larger particles drop out of suspension first
•Shape—The longer alluvium in transported in the
stream, the more rounded and smooth it becomes

21. Sorting after a flood event
(particles settle biggest first, smallest last)
erosion deposition

22. Well-rounded alluvial deposits

23. Stream Flow
How does friction from the channel sides and bottom
affect stream flow?

24. Stream Flow
•Patterns
–Straight
–Sinuous
–Meandering
–Braided

25. Just like a lake, water balance must be maintained
year-round, or the stream will become ephemeral,
only in existence for part of the year.

26. Equilibrium: A Graded Stream
• All factors are balanced (gravity, stream
load, deposition, down-cutting)
• A graded condition is more theoretical than
actual, because equilibrium is so difficult to
achieve—and maintain

27. Which processes happen as a stream
tries to attain a graded condition?
•Valley deepening
•Valley widening and flattening
•Valley lengthening

28. Valley Deepening:
Knickpoint Migration
•Downcutting progresses
upstream, until the
valley is all at one level

29. Valley Deepening: Knickpoint Migration

30. Valley Widening and Flattening

32. Valley Widening and Flattening
The Role of Meanders

33. Meanders and Oxbow Lakes

34. Can you find an oxbow lake?
Where are cutoffs about to form?

35. 35

36. Structures of a Floodplain

37. Structures of a Floodplain
Aggradation—Deposition that raises the stream bed

38. Valley Lengthening:
Headward Erosion

39. Valley Lengthening:
Delta Formation and Distributaries

40. Valley Lengthening:
Delta Formation and Distributaries

41. Valley Lengthening:
Delta Formation and Distributaries

42. Valley Lengthening:
Delta Formation and Distributaries

43. Stream Rejuvenation
Degradation—Erosion that lowers
the stream bed

44. Stream Rejuvenation
and Entrenched Meanders
When uplift is slow, meanders cut down into their
existing channels and cutoffs form arches.

45. Entrenched Meanders