1. Water Cycle - Running Water
Northwest Geology
Chapter 15

4. Streams Streams are the major geological agents
operating on the surface of the land. The chief
focus of streams are
-How water flows in currents
-How currents carry sediment
-How streams break up and erode solid rock
-How streams carve valleys and assume a variety of forms as they channel water downstream

5. Stream Water Flow Begins as sheetflow
Infiltration capacity is controlled by
• Intensity and duration of rainfall
• Prior wetted condition of the soil
• Soil texture
• Slope of the land
• Nature of the vegetative cover
Sheetflow develops into tiny channels
called rills

6. Two types of flow determined primarily by velocity Turbulent flow
Laminar flow
Turbulent flow
Factors that determine velocity
– Gradient, or slope
– Channel characteristics including shape, size,
and roughness

7. Streamflow
Laminar – water particles flow in straight paths giving flat, gentle surface
Turbulent – water moves in an erratic fashion characterized by swirling eddies and whirlpools (white water)

8. Laminar vs Turbulent

9. Streamflow Factors that determine velocity
– Discharge – the volume of water moving past
a given point in a certain amount of time
- Changes from upstream to downstream
- Profile
* Cross-sectional view of a stream
*Viewed from the head (headwaters or source)
to the mouth of a stream

10. Changes from upstream to downstream
-Factors that decrease downstream
• Gradient
• Channel roughness
-Base level and graded streams
(Base level is the lowest point to which a stream can erode

11. Two general types of base level
– Ultimate (sea level)
– Local or temporary
Changing conditions causes readjustment
of stream activities
– Raising base level causes deposition
– Lowering base level causes erosion

13. • Factors that increase downstream
-Velocity
-Discharge
-Channel size

14. Stream erosion
-Lifting loosely consolidated particles by
abrasion
-Dissolution
(Stronger currents lift particles more
effectively)

15. Streamflow Discharge
Discharge is the volume of water that flows past a given point in a given time.
The volume is calculated by multiplying the area water in the stream channel (depth x width) by the velocity (distance/time) of the stream flow.
The dimensions of the stream channel are expressed in feet or meters and velocity is measured in feet or meters per second. Consequently the units of discharge are cubic meters (or feet) per second, i.e. discharge is the volume (cubic meters or cubic feet) of water that passes a given point in one second.

16. Discharge (m3/sec) = channel width (m) x channel depth (m) x velocity (m/sec)

17. Discharge can be calculated by multiplying the area of the stream channel in cross section (width x depth) by the distance traveled in a given time (second).

18. Stream Transport Stream “load”
Dissolved load – dissolved and carried by the water
Suspended load – suspended in and moved by the water
Bed load – moved along the bottom (bed)
Flotation load – light material floating on surface

20. Stream Capacity
Maximum load a stream can support (total sediment load from previous slide)

21. Sediment Deposition • Caused by a decrease in velocity
-Competence is reduced
• Sediment begins to drop out
• Stream sediments generally well sorted
• Stream sediments are known as alluvium

22. Deposition of sediment by a stream
• Channel deposits
• Bars
• Braided streams
• Deltas
• Floodplain deposits
• Natural levees – form parallel to the stream
channel by successive floods over many years

23. Deposition of sediment by a stream
• Floodplain deposits
• Back swamps
• Yazoo tributaries
• Alluvial fans
• Develop where a high-gradient stream leaves
a narrow valley
• Slopes outward in a broad arc

25. Stream Valleys Most common landforms Downcutting toward base level
Often include falls and rapids
Two general types
Narrow and steep
V-shaped

27. Drainage Networks
The pattern of the interconnected networks of streams in an area
Common drainage patterns
Dendritic
Radial
Rectangular
trellis

30. Streamflow Summary
Discharge is the volume of water that flows past a given point in a given time.
The USGS has over 7,000 stream gaging stations.  Data from over half of which are available on-line
Stream gages measure the depth of water in a stream channel and the velocity of flow
The recurrence interval is the average time in years between floods of the same size
A hydrograph  illustrates discharge over time

31. Stream velocity may be measured quite simply
Stream velocity may be measured quite simply.   The technique is to drop a floating object in the stream, time its travel along a meter stick several times, and average the readings

32. Stream velocity varies within a stream channel
Stream velocity varies within a stream channel.  Maximum velocity occurs on the outside of channel bends; minimum velocity occurs on the inside of bends.

33. Erosion occurs on the outer banks of streams (cut-banks) where velocity is greatest and deposition occurs on inner banks (forming point bars) where velocity is least. The stream channel slowly migrates across the valley floor in the direction of the erosion to form long, looping bends termed meanders.

36. Fluvial features

37. Oxbow lake

38. Factors of stream velocity that control erosion
Gradient
Channel shape, size roughness
discharge

39. Summary of Stream Velocity
Stream velocity varies around curves in stream channels
Velocity is greatest on the outside of curves, least on the inside
Erosion occurs in areas of higher velocity forming cut-banks on the outside of stream curves
Deposition in regions of low stream velocity forms point bars on the inside of curves
Erosion and deposition cause stream channels to migrate laterally
Velocity decreases as the  length of the wetted perimeter (channel banks and bed) increases
A floodplain is a broad, flat plain adjacent to a stream channel