Date post: | 16-May-2015 |
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CH. 9 - Running Water
Two sources of energy:
1) Internal energy - heat from core & radioactivity
Powers:a) EQ’sb) Volcanoesc) Mountain building
Two sources of energy:
2) Solar energy
Powers:
a) ocean circulation
b) weather & climate
c) hydrologic cycle
Hydrologic Cycle
Includes: - running water- ground water - glaciers- atmosphere (water vapor)
Distribution of Earth’s Water
97% ocean water
3% freshwater/brackish water
(Fig. 9.1)
Running Water (Streams) = 0.0001% of hydrosphere
Function of streams is to drain the land
- 36,000 km3 water drains annually
- single most important agent changing landscape
Running Water
Source is precipitation (ppt.)
~25% total ppt. becomes surface water
- depends on infiltration capacity
Infiltration capacity depends on:
1)
2)
3)
4)
5)
Drainage Basin
Total land area drained by river and its tributaries
Tributary = smaller stream flowing into a larger stream
Ex: Kickapoo Creek into IL River
Drainage Basin
Mississippi River = 3,222,000 km2
Amazon River = 5,778,000 km2
Drainage Basins
Empty into oceans
Exception: Great Basin in Nevada
- internal drainage basin (no streams flow out)
Drainage Divide
High point dividing adjacent drainage basins
Ex: Continental Divide in Rocky Mtns.
Stream Order
First-order = streams with no tributaries
Second-order = two first-order streams unite
(only first-order streams as tributaries)
Stream Order
Third-order = two second-order streams unite
- can have first- and second-order streams as tributaries
Streams
Three main areas:
- zone of erosion
- zone of sediment transport
- zone of deposition
Streams
Ability to erode/deposit material is function of its velocity
Velocity = rate of flow = meters
second
Stream Velocity Factors:
1) Gradient – slope of stream channel
Slope = rise
run
= amount of elevation change
distance measured
Gradient
Varies considerably from:
a) one stream to another
b) along the course of any given stream
Ex: Gradient changes in Missouri River (maps in lab)
Stream Velocity Factors:
2) Channel characteristics
a) Shape – controls amount of water in contact with channel
Contact with channel causes velocity to (slow down, speed up).
Channel Characteristics
b) Size – larger channel is more efficient
Why?
Channel Characteristics
c) Roughness –sediment lining stream channel
Boulders = rough = more turbulent = slower
Clay/sand = smooth = faster
Stream Velocity Factors:
3) Discharge
Amount of water flowing past a certain point in a given time period
Stream Discharge
Discharge = channel x channel x velocity width depth
Discharge = meters x meters x meters second
Discharge = meters3 = cms second
Stream Discharge
Mississippi RiverDrainage basin = 3,222,000 km2
Discharge = 17,300 cmsAmazon River Drainage basin = 5,778,000 km2
Discharge = 212,400 cms
Stream Order
As stream order increases:
- discharge __________
- gradient __________
- velocity __________
- channel dimensions ________
Stream flow
1) Laminar flow – straight line
- indicates slow velocity or smooth stream channel
Stream flow
2) Turbulent flow
- indicates fast velocity or rough stream channel (rapids)
- increased erosion - more sediment in suspension
Longitudinal Profile
Cross-sectional view of a river from headwaters to mouth
Changes:a) Gradient decreases from head
to mouth b) Discharge increases towards
mouth of river
Base level
Lowest elevation a stream can erode down its channel
Ultimate base level = sea level
Local base level:
- lakes, resistant rock layers, larger streams, reservoirs
Base Level
Changes in base level causes changes in stream activities
Base Level
Ex 1: Building a dam
- creates new local base level
Changes upstream:
Changes downstream:
Base Level
Ex. 2: Draining a lake
- stream channel cuts down to the next local base level
Stream Transport
1) Bed load – material in contact w/channel bottom
- moves by sliding, rolling, saltation (jumping motion)
- moves intermittently
Stream Transport
2) Suspended Load - usually fine sand & clay-size
particles- visible sediment (muddy look of
rivers)- largest amount of material
carried by streams
Stream Transport
3) Dissolved Load
- invisible
- transported regardless of stream velocity
- precipitates only when stream chemistry changes
Dissolved Load
Measured in parts per million (ppm)
~4 billion metric tons supplied to oceans annually
Streams’ ability to erode/deposit depends on:
1) Capacity
- maximum amount of sediment stream can transport
- directly related to discharge (volume of water flowing)
Streams’ ability to erode/deposit depends on:
2) Competence – largest sediment size stream can move
- as velocity doubles, competence quadruples
Flood stages
Greatest power of streams to erode & transport material
Capacity:
Competence:
Types of stream channels
1) Braided
- channel crosses back & forth
- shallow stream channel with large sediment load
Braided streams
Sediment load is deposited quickly
Caused by:
a) abrupt decrease in gradient
b) decrease in discharge (drought)
Types of stream channels
2) Meandering
- river is confined to one channel
- river channel is curved
Curves = meanders
Alluvium
Stream-deposited sediment
- well-sorted by size
As velocity decreases, heaviest sediment is deposited first
Depositional Features
RapidsPoint barsFloodplainsNatural leveeBack swampYazoo tributary
Depositional Features
Delta
Alluvial Fan
Erosional Features
WaterfallsV-shaped valleysCutbanksMeandersMeander cutoffs Oxbow lakes
Erosional Features
Meander scar
Incised meander - meanders enclosed in steep valley walls
Stages of stream valley development
Stages of Stream Valley Development
Early stage:- stream is well above base level- downcutting is main erosional
work- narrow V-shaped valley w/no
floodplains
Early stage
- relatively straight course
- rapids & waterfalls may be common
Stages of Stream Valley Development
Middle stage:- stream closer to base level- lateral erosion is major
erosional work- floodplain created & enlarged
Middle stage
- meanders
- cutoffs & oxbows may be present
- floodplain defined by meander belt
Stages of Stream Valley Development
Late stage:- stream close to base level- very wide floodplain- primary erosional work =
reworking floodplain sediments
Late stage
- meanders, cutoffs, oxbows, meander scars
- natural levee is well developed
- backswamps, yazoo tributaries
Rejuvenated Stage
River near base level is uplifted
- downcutting dominates
- incised meanders
Ways to Lengthen Valleys:
1) Deposition at mouth of river
Ex: delta
2) Headward Erosion
- erosion cuts into upland area at the head of the river
Stream Piracy
Diversion of one stream channel by headward erosion of another stream
Stream Piracy
Two streams on opposite sides of a divide:
Stream w/ steeper gradient cuts headward faster
- cuts across divide and captures water from slower stream