1. Rivers
Yukon and Charley Rivers

2. Topics 1. Introduction to fluvial processes 2. Types of channels
3. Large Meandering Rivers
4. Understanding & Interpreting fluvial forms

3. 1. Introduction to fluvial processes
Streams “work” by eroding, carrying and depositing sediment

4. Solid load: suspended load & bedload

5. Suspended Load Little Colorado River

6. Large bedload, carried during monsoon rains in Nepal
Constant suspended load of glacial flour (ground up rock)

7. Classroom Resources Movies showing bedload transport of gravels
Movie showing sand transport & ripple formation

8. Classroom Resources
Load_Fluvial.swf – animation showing types of load carried by streams

9. Classroom Question
In Lord of the Rings (popular with students), Isildur died, slipped from his finger into the big river, and the ring stayed put for over a thousand years -- before it was found by Gollum.
How is this possible, given that the ring can be seen in bedload?

10. Answer comes later - when students learn about oxbow lake formation – perhaps where ring found
Image of
Mississippi
River

11. Also dissolved load

12. Suspended Load
Khartoum
Dissolved Load
Suspended Load

13. Manaus
Dissolve
Load
Organic
Acids!
Manaus
Suspended Load

14. Erode (degrade)? Carry (transport)? Deposit (aggrade)?

15. Erode? Carry? Deposit?
200 cm/s ~ 4.5 mi/hr

16. Elements Of Channel Formation
Elements of Channel Formation – One key function of streams is to move the products of erosion to lower, downstream locations. Erosion begins in a Source area that can involve eroding bedrock or reactivating earlier transported material. Sediment from the Source is then Transported through the stream system The channel that is formed during this Transportation process will reflect the material balance that exists. When the system has no net loss or gain of sediment (this condition is often referred to as being “In regime”), the channel will usually be relatively stable vertically but can experience considerable lateral shifting. This lateral shifting is most common in meandering streams. Finally, the sediment must be deposited somewhere within the system. This deposition often occurs just upstream of a site where Base Level Control occurs. Normally, braiding and multiple channels form in the depositional area and the stream has unstable channels.
Diagram used with the permission of Geomax, P.C., Spokane, WA

17. Erosion in headwaters
Transport in middle
Deposition closer to ocean

18. High gradient (mountain) streams – erode
and carry large bedload (high competence)

19. High competence load creates potholes & other evidence of abrasion

21. Middle – mixture of erode, transport, deposit, and for much of the year – like this
Depends on flood or not

22. Lower end - deposit Mississippi River delta has very high capacity
(lots of sediment,
Just small competence)

23. These are generalizations that work for larger system and not smaller pieces

24. Organisms influence streams, especially at reach & smaller areas See Online Video

25. Flow Velocity Relationships
Velocity Relationships – Sediment transport is highly sensitive to local flow velocity (turbulence). For example increasing flow velocity from 4.5 feet per sec. to 9 feet per sec. will increase the water’s transport potential from a 1 lb. rock to a 90 lb. rock.
Diagram used with the permission of Geomax, P.C., Spokane, WA

26. Note: always concave upward profile – characteristic of streams
Even these small washes

27. Base Level & Grade Grade – concave upward profile
The ability of a stream to erode is based on velocity of water. Velocity is proportional to slope.
At a sufficiently low slope, streams will run without eroding: this is called the base level. The ultimate base level is sea level, although local base levels exist, since flow paths are seldom one consistent slope.

28. Profile – concave upward
Base Level
Base level is a short-term concept. Change in sea level, for instance, can either increase or decrease slope of a stream profile.
Dams create artificial local base levels, and in so doing force steeper slopes (and hence erosion) downstream.

29. Effects Of Dams From FEMA
Effects of Dams -- When a dam is placed on a stream, the sediment transport capability of the stream is disrupted. The dam becomes a local base level control. As the water slows at the head of the dam, any sediment that was being transported tends to be deposited and a delta is formed.
The water released immediately downstream from the dam contains considerably less sediment than it did before the dam was built. This released flow is often termed “hungry water” because it usually triggers local erosion of the bed and banks of the channel below the dam. Most dams also tend to reduce peak flow by storing water during high runoff periods.
Unregulated Side Drainages that enter the dammed stream also carry sediment loads. Prior to dam construction, these sediment loads normally were flushed down the main stem during peak flows. When the peak flows are reduced by the dam, the sediment coming from the side drainages cannot be flushed away as efficiently. When this happens an alluvial fan tends to form at the mouth of the unregulated side drainage. These alluvial fans then pinch the main stem flow and cause a slackwater area to develop above the fan and rapids to form in the pinched section.
An additional problem develops if the dam is used as a peaking power facility. Peaking power facilities require the flow from the dam to be rapidly changed to generate power at critical times (usually in the morning at breakfast and in the evening at supper time). These rapid changes of flow cause excessive bank seepage forces to develop when rapid drawdown occurs. These seepage forces coming out of the bank often destabilize the banks and sloughing can occur.
Diagram used from FEMA presentation with the permission of Geomax, P.C., Spokane, WA
From FEMA

30. Classroom Resource
Dam Break at Kaui – showing erosion when discharge and velocity increases

31. When base level drops, every tributary incises because streams are steeper and have more energy

32. When Grand Canyon formed, it “entrenched” meanders
Base level dropped ~ 5 million years ago, freezing in place the old stream patterns

33. Classroom Resource
Spinaround (QTVR) at Goosenecks of the San Juan River

34. 2. Types channels 3 basic types of channel patterns: Straight Channels
Meandering channels
Braided Channels

35. Straight – in nature, requires a weakness (fault, jointing) for stream
Straight – when you see, usually from human activity (channelized, Nasca, Peru)

36. Meandering
Meandering rivers typically contain one channel that winds its way across the floodplain. As it flows, it deposits sediment on banks that lie on the insides of curves (point bar deposits), and erode the banks on the outside of curves.

37. Point Bar Cut Bank
Point Bar Cut Bank
Asymmetrical

38. Meandering Streams – Streams that are vertically stable normally develop meanderings patterns. Valleys usually are wide and flat. The meander loops shift back and forth across the floodplain as the system transports the sediment load downstream that enters the valley. Meandering always flow in alluvial valleys, tend to armor their beds and create slack water deposits on the floodplain. Diagram used with the permission of Geomax, P.C., Spokane, WA

39. Braided Channel

40. Braided Rivers exhibit numerous channels that split off and rejoin each other to give a braided appearance. They typically carry coarse-grained sediment down a steep gradient

41. Classroom Resource Lab showing movement
among braided channels
“Spin around” – can see braided stream

42. Braided – carries mostly bedload, so stream spreads out to bring high velocity close to bed
Meandering – carries much suspended load, so stream narrows to carry load more efficiently

43. Can get very complicated

44. 3. Large Meandering Rivers

45. Meandering stream, Phnom Penh, Cambodia
Oxbow lake
Point bar
Cut bank

46. Zooming into a Reach
Riffle
Pool

47. Poor Bridge Entry Problems -- When meandering streams “fold in” on roads that cross the floodplain as shown in the previous slide, a number of problems develop. The sharp curve that occurs causes the inside of the bend to become depositional. When this happens, it is common to find a loss of conveyance dropping significantly through the bridge opening. This loss of conveyance can then cause water levels above the bridge/road crossing to become higher during periods of peak flow.
Additionally, upstream bank erosion often accelerates and the bridge abutment on the outside of the bend is subjected to increased flow turbulence and can be undercut by excessive bed scour.
Diagram used with the permission of Geomax, P.C., Spokane, WA

48. Bank Erosion at Cut Bank

49. Bank Erosion Newaukum River (WA)

50. Classroom Resources

51. Point Bar Deposit

52. Point bars

53. Floodplains

54. A Floodplain
These are satellite images before and during Summer, 1993 floods of the Mississippi river north of St. Louis.

55. Classroom Resource
Animation of before and after 1993 floods

56. Floods
In natural streams, floods occur periodically when seasonal discharges grow extremely large and waters exceed channel banks.
The first thing that happens after water goes over bank is the velocity drops, and sediment is deposited. This makes natural levees along channels.

57. Yazoo Stream

58. Classroom Resource
NaturalLeveesBuilt.swf – animation showing building of natural levees from successful flooding by a meandering stream

59. Societal Problem Living below the river!
Cities grow & obligation to protect
Classroom Resource Folder – movie of 1991 Flooding

60. Certainty: levee will always break. Only question: when

61. Tendency to increase meander amplitude over time

62. Classroom Resources
Animated maps showing gradual change and also sudden shifts (avulsion)

63. Tendency towards avulsion (sudden change in channel)
Sacramento R.

64. Oxbow lakes

65. Classroom Resources

67. Back to the student hook: perhaps an avulsion occurred soon after the ring slipped through Isildur’s ringer?

68. 4. Understanding & Interpreting fluvial forms (or something to do on a family vacation)

69. 4. Understanding & Interpreting fluvial forms
Basic Stream Valley between Interfluves

70. V-shaped valley, Amazon Headwaters
Interfluve (ridge)
V-shaped valley, Amazon Headwaters
Interfluve (ridge)

71. Eroding river valleys – v-shape

72. Knickpoints

74. Rivers work to destroy knickpoints
1st. Erode down & create v-shaped valleys
2nd. Push knickpoint up stream by undercutting
Niagara Falls

76. Classroom Resources Reconstruction showing Niagara Falls Retreat
Flume Movie

77. Fill Stream Terrace: Stream cuts through its old fill

79. ASU is on a Fill Terrace

80. Terrace Breaks Used to Irrigate

81. Troy Pewe

82. Why does a stream incise?
Uplift of mountains
Lowering of sea level (during glaciations)
More discharge (extreme flooding events) –creates arroyos in the southwest

83. Terrace – created by repeated uplift of mountains
Terrace – created by repeated uplift of mountains. Strath: stream erodes floodplain on bedrock and then gets additional energy to erode into its old floodplain

84. Strath Terrace – can see the erosion of the bedrock by the stream (not eroded strata)

85. Delta: Nile River
Wave-driven longshore drift redistributes sediment along the coast.

86. The Mississippi Delta
Jetties have been built and dredging occurs to allow large vessels to navigate the delta channel
Note the large amounts of sediment that surrounds the delta
A bird’s foot delta located in Louisiana adjacent to the Gulf of Mexico

87. Deltas
Distributaries in deltas periodically shift to find the most efficient path to the sea.
Delta-related wetlands are under stress from sediment starvation and lack of natural flooding.

88. The Atchafalaya Delta
This is one of the few areas where the Louisiana coastline is growing
This delta is considered to be the Mississippi’s largest distributary
The Atchafalaya diverts large quantities of water from the Mississippi

89. Tide-Dominated Delta Indus Delta Tidal channels and flats
Tidal channels widen toward sea

90. The Irrawaddy River Delta
Sediment plumes
mangrove forest
Fertile mud and sand have been deposited here during the last 2 million years
This river delta in Burma is one of the world’s great rice producing regions

91. Wave- & Tide-Influenced Delta

92. Online Videos

93. Online Visualizations
Flood Visualizations
River Systems: Process and Form
Waterfall Formation
River Erosion, Deposition, and Transportation Visuals
Deltas

94. Imagery seen in this presentation is courtesy of Ron Dorn and other ASU colleagues, students and colleagues in other academic departments, individual illustrations in scholarly journals such as Science and Nature, scholarly societies such as the Association of American Geographers, Geomax presentations for FEMA, city, state governments, other countries government websites and U.S. government agencies such as NASA, NPS, USGS, NRCS, Library of Congress, U.S. Fish and Wildlife Service USAID and NOAA.