Rita Haberlin

1
The Work of Streams

• Rita Haberlin
• College of Alameda

2
Q. What is the most important agent shaping the
landscape?

• weathering
• mass wasting
• surface water runoff
• sheetwash
• rill erosion
• streamflow

A. Running water
3
Where do streams get their water supply?

• Surface water runoff
• Ground water (base flow)

4
How does precipitation find its way to the ocean?
5
Q. What is the most common pattern made by
streams?

• The most common pattern is dendritic
• Streams form a branched network
• This is called a drainage system

6
Q. What is a drainage basin?

• The total area drained by a river and its
  tributaries
• A drainage basin is defined by divides
• Divides separate one drainage basin from another

7
Drainage Basin and Divide
8
Q. What is the ultimate limit of stream erosion?

• The limiting level for a stream is base level
• Sea level is base level for most streams
• Sometimes a lake forms a local base level

9
Q. What is the work of streams?

• Erosion
• Transport
• Deposition

10
Q. What determines the ability of a stream to do
work?

• Velocity - speed of flow

11
Q. What determines the velocity?

• Gradient - slope of the channel
• Channel shape and roughness
• Sediment load
• Discharge - amount of water flowing past a point
  in a given unit of time

12
Gradient

• Gradient usually decreases downstream
• Does this mean it will flow more slowly?

13
Which channel is most efficient?
Channel Shape
14
Channel Roughness
All things being equal, water in a rough
boulder-filled channel should flow _____?
15
Channel Cross-section

• On straight sections, rivers flow fastest in the
  middle
• Where does a river flow fastest on a meandering
  stream?

16
Velocity in Meandering Rivers

• Streams flow faster on the outside or inside
  bend?
• Outside

17
What is Discharge?

• Amount of water flowing past a point in given
  time
• What factors influence the amount of discharge?

18
Discharge in World Rivers

• Precipitation and size of the drainage basin

19
Sediment load

• A streams velocity determines its competence to
  carry load
• If velocity doubles, the streams competence
  increases four times
• During floods, streams carry larger-sized
  particles and a larger total load

20
Discharge

• Amount of water flowing past a point in given
  time
• Units of measure is Volume/Time
• cubic feet per second
• cubic meters per second
• Discharge is determined by the
• width x depth of stream x velocity
• Mississippi - 611,000 cubic feet per second

21
How do streams erode?

• Hydraulic action or scour (sheer force of water)
• Abrasion
• Dissolution

22
What kind of erosion carved these potholes?
23
How do streams transport their load?

• 1. In suspension (suspended load)
• 2. In solution (dissolved load)
• 3. By traction, rolling, and dragging (bed load)

24
When do streams deposit their load?

• When the velocity decreases
• What would cause velocity to decrease?
• reduced discharge
• lower gradient
• inside bend of a river
• higher base level
• increased sediment load

25
How do streams change as they reach sea level?

• Discharge increases as tributaries join the
  stream
• Width and depth of channel increase
• Sediment load increases but particle size
  decreases
• Velocity increases slightly

26
Streams show three stages of valley shape from
mountains to the sea

• Youth
• Maturity
• Old Age

27
Stage 1. Youth
28
Characteristics of Youthful Mountain Streams

• Steep irregular profile
• Waterfalls, rapids, and plunge pools
• River is deepening its channel
• Cutting into bedrock
• Potholes
• V-shaped cross-section

29
Waterfalls
30
Large Boulders as Bed Load
31
Stage 2. Maturity
32
Lateral Erosion

• Erosion is greatest on the outside bend
• cut-banks
• Deposition on the inside bend
• point bars

33
Lateral Erosion

• Meanders
• migrate downstream
• widen the valley
• form cutoffs
• Create oxbow lakes

Oxbow lake
34
(No Transcript)
35
Stage 3. Old Age
Backswamp
Flood plain
Oxbow lake
Yazoo stream
Natural levees
Alluvium
36
Natural Levees

• Built by floods
• Depositional feature
• Parallel to the stream Channel on both banks

37
Sacramento River
38
(No Transcript)
39
Which Areas Flood Most in U.S?
40
Mississippi River Basin

• Worst flood in U.S. history in 1993
• 17,000 sq miles
• Heavy rainfall in June and July
• Upper Mississippi Basin to St. Louis

41
Excessive Rainfall

• Artificial levees cannot contain floods
• Give false sense or security

42
Floods in St. Louis, 1993
Summer 1988
Summer 1993
43
Rivers drop their load when they enter the ocean
and form a ?
44
RejuvenationSometimes a river goes through a
renewed period of erosion

• What causes rejuvenation?
• Uplift
• Increased discharge
• Decreased load
• Lower sea level

45
How Rejuvenation Affects Rivers

• Rivers cut down into bedrock to form entrenched
  meanders

46
How Rejuvenation Affects Rivers

• Rivers cut down through their valleys

Terraces
47
How Do People Change Rivers?

• Deforestation
• Channelization
• Urbanization
• Construction
• Paving
• Dam building

48
(No Transcript)
49
How does development change streams?

• Natural stream
• Suburban stream
• vegetation removed
• increased sediment load
• Urban stream
• concrete and asphalt
• less water sinks in
• more runoff
• greater channel erosion
• flashfloods

50
Urban Stream Floods
51
(No Transcript)
52
How Do All These Changes Affect Rivers?

• Less infiltration so less ground water
• More runoff
• More flooding in form of flash floods
• Greater channel erosion

53
Central Valley Floods
54
Stream Pollution