1. The Work of Streams

2. The Work of Streams
Streams generally erode their channels lifting loose particles by abrasion, grinding, and by dissolving soluble material.

3. The Work of Streams
Streams transport materials of erosion in three ways:
In Solution (dissolved load)
In Suspension (suspended load)
Scooting or Rolling along the bottom (bed load)

4. Dissolved Load
Most of the dissolved load finds its way into streams via groundwater.
Usually expressed as dissolved material per million parts of water (parts per million).

5. The Work of Streams
Streams deliver almost 4 billion metric tons of dissolved substances to the oceans each year.
Photo by R. Jowsey

6. Suspended Load
Most streams carry the largest part of their load in suspension.
The visible cloud of sediment suspended in the water is the most obvious portion of a stream’s load.

7. Streams usually only carry sand, silt, and clay in this fashion.
Suspended Load
Streams usually only carry sand, silt, and clay in this fashion.
Higher velocity in a stream may allow it to carry larger particles in suspension.

8. Bed Load
Bed load is that part of a stream’s load that is too large to be carried in suspension.

9. Competence and Capacity
The ability of a stream to carry a load is determined by two factors:
Competence
Capacity

10. Competence
Competence of a stream measures the largest particles it can carry.
A streams competence increases with velocity.

11. Capacity A stream’s capacity is the maximum load it can carry.
Capacity is directly related to a stream’s discharge.
The greater the volume of a stream, the greater its capacity for carrying sediment.

12. Click for Deposition Animation
As a stream’s velocity decreases (slows down), its competence decreases and sediment begins to drop out, largest particles first.
Click for Deposition Animation

13. Deposition
Deposition occurs as streamflow drops below the critical settling velocity of a certain particle size.

14. Sorting
Stream transport separates solid particles of various sizes, large to small.
This process is called sorting.
It explains why particles of similar size are deposited together.

15. Alluvium The sorted material deposited by a stream is called alluvium.
Many different depositional features are made of alluvium.

16. Deltas
A delta is an accumulation of sediment formed where a stream enters a lake or ocean.

17. Natural Levees
A natural levee is a ridge made up mostly coarse sediments that parallels some streams.

18. Natural Levees
A natural levee forms when a stream repeatedly overflows its banks.

19. Narrow Valleys
A narrow v-shaped valley indicates that a stream’s primary work has been down-cutting towards base level.

20. Narrow Valleys
The Yellowstone River is an excellent example of a narrow valley.
Rapids and waterfalls are the predominant features of a narrow valley.
Photo by R. Jowsey

21. Narrow Valleys
Photo by R. Jowsey
Photo by R. Jowsey

22. Narrow Valleys
Photo by R. Jowsey

23. Wide Valleys
Once a stream has cut its channel closer to base level, downward erosion decreases.
More of the stream’s energy is then directed from side to side.
The result is a widening of the valley as the river cuts away at first one bank and then the other.

24. Floodplain
The side to side cutting of a stream eventually produces a flat valley floor, or floodplain.

25. Floodplain Streams that flow on floodplains move in meanders.
A meander is a loop-like bend in the course of a stream.

26. Meanders Once a meander begins to form, it gets larger.
Most erosion occurs on the outside of the meander.

27. Meanders
The outside bank in a meander is sometimes called the cut bank.

28. Check for Understanding
Why is the cut bank (outer bank) the area of greatest erosion in a meander?
The cut bank is the zone of greatest stream velocity.

29. Oxbow Lake
An oxbow lake is a crescent-shaped (often temporary) lake that is formed when a bend in a river is cut off from the main channel by the forces of erosion.

30. Oxbow Lake
An oxbow lake will slowly be created as soil erodes and re-deposits, changing the river's original course.

31. Oxbow Lake Oxbow lake formation - video demonstration (click left).

32. Independent Assignment
Use your notes and your knowledge of biology to answer the questions on your worksheet.

33. References Adapted from:
Tarbuck & Lutgens (2009). Earth Science. Prentice Hall, Boston.