1. Erosion and Transportation
River Systems II
Erosion and Transportation

2. Daily Question
Which of the following effects does urbanization have on stream systems?
Decrease risk of water pollution
Increase in discharge
Increases flooding intensity and frequency
Increases erosive ability of the stream
Increase in infiltration to ground water

3. Urbanization Effects Decrease infiltration – Increases runoff
Produces Higher Peak Discharge
Produces shorter lag time
Produces more floods
Higher discharge = increased ability to erode

4. Erosion Removal and transport of rock, sediment, soil
Running water is the major cause of erosion
Source of Materials in a stream
Mass Wasting
Lateral erosion of banks
Downward erosion of stream channel
Headward erosion
Sheetflow
Chemical Weathering

5. Stream Dynamics – Sediment Load
Bed Load
Suspended Load
Dissolved Load

6. Sediment Load – Bed Load
Particles to large to be lifted into suspension
Sediment moves along stream bed
Particles move by sliding, rolling, or saltating (short leaps)
Accounts for 10 to 25% of sediment load
Agent of downward erosion

7. Sediment Load – Suspended Load
Light sediment lifted above stream bed by current
Mostly silts and clays
Accounts for most of stream load ( 60%)

8. Sediment Load – Dissolved Load
Chemical ions produced from chemical weathering of minerals
Ca2+, Cl-, Mg2+, SO42-, Na+, K+, HCO3-, etc.
Acquired from groundwater, sheetflow, or dissolving rock along a stream’s course.
Generally  10%, but can be up to 50% of sediment load.

9. Effects of Transport on Particles
Rounding
caused by abrasion
Increases with distance particle is transported
Sorting
Varies with the competence of the stream
As competence decreases large particles are deposited

10. Controls of Sediment Load
Capacity
The amount (weight) of sediment a stream can carry
It is a function of discharge (velocity)
Velocity  (proportional to) Capacity 3 to 4
Example, if velocity doubles (2) – capacity increases between 8 (23) to 16 (24) times
Competence
The maximum particle size a stream can transport
Function of velocity and fluid density
Velocity  (proportional to) Competence2
Example, if velocity doubles (2) – competence increases 4 (22) times
As stream density increases with more suspended and dissolved sediment, competence increases

11. Fig

12. Velocity’s role in Erosion
Threshold Velocity – minimum velocity required to move grains of a certain size

13. Fig

14. Deposition of Material
Occurs when streams lose velocity (competence decreases)
Velocity decreases due to
Lower gradients
Flow into still or slow moving waters (reservoirs, lakes, etc.)
Changes in channel shape (widening or narrowing)

15. Meanders - Cut Banks & Point Bars
Fig

17. Meander Development Meander loops migrate laterally and downstream
As meanders migrate and streams continues to erode and deposit sediment, a meander can be cutoff creating an oxbow lake.
What would happen to the streams gradient?

18. Longitudinal Profile
Cross-sectional image showing the variation in a stream’s elevation along its length

19. Base Level
The downward limit of stream erosion; lowest elevation to which a stream can erode its channel
Ultimate Base Level – sea level
Local Base Level – lakes, resistant layers of rock, reservoirs, etc.

20. Effects of Changes in Base Level
Changes in base level change the energy of the system, which changes the velocity of the water.
Consider the consequences of constructing a dam on a river that has a large stream load, such as the Yellow River, China. Assume the dam and its reservoir are located about 2/3 of the way down the river. How would stream conditions be altered above and below the dam and its reservoir? What would be the implications for erosion, transport, and deposition?