Streams Nancy A. Van Wagoner Acadia University Distribution of Earth's water n What are the...
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![Page 1: Streams Nancy A. Van Wagoner Acadia University Distribution of Earth's water n What are the percentages? -Oceans - Glaciers - Groundwater - Lakes and.](https://reader036.fdocuments.in/reader036/viewer/2022062719/56649ef25503460f94c03ce7/html5/thumbnails/1.jpg)
Streams
Nancy A. Van Wagoner
Acadia University
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Distribution of Earth's water
What are the percentages?- Oceans - Glaciers - Groundwater - Lakes and Rivers - Atmosphere
97.5%
1.8%
0.63%
0.02%
0.001%
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Importance of fresh water resources
Life on earth
Carve landscape
Transportation
Recreation
Electricity
FISHERY
Irrigation-agriculture
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Hydrologic Cycle (Fig. 11.1)
Continuous movement of water between the oceans, the atmosphere and land
The system is sun-powered. Values in 1000's of km3/yr.
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Streams and their characteristics stream = an channelized flow,
regardless of size river = large stream fed by smaller
tributariesflood plain
banks
bed
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Characteristics Continued
wet year round versus intermittent
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Work of Streams
erode material transport material deposit material
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Stream Velocity
Important because it is directly related to the ability of a stream to do the work of streams.
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Velocity Distribution
see figure the velocity distribution is controlled by
the frictional drag of the- air- bed- banks
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Top view
Maximum velocity at the centre
Side view
Maximum velocity is just below the surface
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Factors Controlling Velocity:
slope or stream gradient channel size and shape roughness of the bed and banks discharge
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Factors Controlling Velocity
Stream gradient = slope of stream- drop/unit distance m/km
- slope is directly proportional to velocity• streams cutting newly uplifted areas: high
gradient/velocity• Mississippi - lower gradients < 1 m/km
Metersdrop
Kilometer of horizontal distance
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Factors Controlling Velocity
Channel size and shape- determines amount of water in contact with
the bed and banks- The most efficient channel: least perimeter
for cross sectional area- The most efficient channel space: large
semicircular shape- DIAGRAMS: 3 streams with same cross
sectional area, different perimeter
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h
w
1unit
10 units
2 units
5units
2.5
Cross-sectional area
Perimeter
h x w
R2/2
10
10
10
12
12
7.5
h + w + h
R
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Factors Controlling Velocity
Roughness of the Channel- refers to the size of particles lining the
channel- increasing roughness, increases frictional
drag
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Factors Controlling Velocity Discharge
- volume of water flowing past a given point/unit time- Q = cross sectional area x velocity = m3/sec
• Q = A x V
- Flood: more water:• size of channel increases: velocity increases therefore Q
increases
- Drought: deposition• size of channel decreases therefore frictional drag
increases, and velocity decreases therefore Q also decreases
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Changes downstream: (fig. 9.5) streams adjust to maintain a balance between
all factors that govern their flow Near Headwaters:
• least discharge
• steepest gradient
• narrow, shallow channel
Downstream:• number of tributaries increases therefore discharge
increases– recall Q = A x V
– V would increase drastically if A and gradient were not adjusted, therefore
• stream widens and deepens , increasing A
• gradient decreases
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Sediment size related to:
stream energy transport distance
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Stream Erosion - Three Processes
see fig. 11.5 1. Hydraulic Action: erode by lifting
unconsolidated material due to impact of water
2. Abrasion: sand paper effect due to sediment in water
3. Solution: minerals dissolved in water
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Sediment Transportation (fig. 11.5) 1. Dissolved load
- carried in solution- acquired mostly from groundwater- ~ 20 % of total load
2. Suspended load- most of load- fine-grained: silt and clay
3. Bed load- rolling and sliding- saltation
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Ability of a stream to erode and transport material is
established by:
1. competence: maximum size - competence is proportional to velocity
2. capacity: maximum load - capacity is proportional to discharge
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Base Level (Impt. Concept)
ultimate point to which a stream can erode its channel
Ultimate base level = sea level temporary base level examples:
- lake, reservoir, internal drainage basin Any change in base level causes the
stream to adjust its channel. See diagram (fig. 11-13)
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Deposition and Depositional features
deposition occurs when velocity decreases
competence decreases and some suspended sediments settle
out
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Alluvium
unconsolidated sediment deposited by a stream
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Depositional Features Delta: form where stream enters ocean or
lake; velocity suddenly reduced; result = deposition of a triangular wedge of sediment called a delta (fig. 11-10 and 11-11)- Important! The shape and extent of a delta is a
balance between:• A. deposition rate of deltaic sediment, and• B. rate at which sediment is removed by erosion
- Large rivers tend to have large deltas
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Depositional Features Alluvial Fan -Fan-shaped alluvial deposit at
the base of a mountain or hill- Forms where mountain stream reaches the plain- Gradient abruptly lowered - rapid deposition occurs
Very unstable for building development due to:- poorly consolidated materials- shifting of distributary channels
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Depositional Features
Natural levees and flood plains- Periodically, rivers overflow their banks
and deposit sediment- Natural levees can be 6 m+ high. - The area behind the levee is poorly
drained = back swamp.
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Stream Valleys: Two General Types
1. narrow v-shaped 2. wide valleys with flat floors
- there is a gradation between the two types
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Type of valley gives indication of the amount and
type of work the stream is doing:
v-shaped: down cutting wide/flat: lateral erosion
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narrow valleys: characteristics
rapids waterfalls steep gradient; active faulting/uplift, rock
resistance above sea level
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wide valleys: characteristics
Form when stream channel is cut close to base level;
down cutting - less dominant; energy is directed side to side
Result = widening valley, formation of a flood plain
River confined to channel except during flood.
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Meandering Streams Type of stream that flows on broad flood plains. Meanders always changing position because the
maximum velocity is directed outward toward the bank.
Result = meanders move sideways and slightly downstream
If one meander moves downstream slightly faster than another then:- Cut off meander and oxbow lake may form- Eventually leaves a meander scar
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Braided streams
Most streams are braided to some extent.
Very common in arid regions; - lots of unconsolidated sediment; - little water- low frequency/high volume rainfall
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Drainage patterns
give an indication about geology- dendritic- trellis- radial