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
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?)
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
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
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