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Streams
Nancy A. Van Wagoner
Acadia University
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%
Importance of fresh water resources
Life on earth
Carve landscape
Transportation
Recreation
Electricity
FISHERY
Irrigation-agriculture
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.
Streams and their characteristics stream = an channelized flow,
regardless of size river = large stream fed by smaller
tributariesflood plain
banks
bed
Characteristics Continued
wet year round versus intermittent
Work of Streams
erode material transport material deposit material
Stream Velocity
Important because it is directly related to the ability of a stream to do the work of streams.
Velocity Distribution
see figure the velocity distribution is controlled by
the frictional drag of the- air- bed- banks
Top view
Maximum velocity at the centre
Side view
Maximum velocity is just below the surface
Factors Controlling Velocity:
slope or stream gradient channel size and shape roughness of the bed and banks discharge
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
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
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
Factors Controlling Velocity
Roughness of the Channel- refers to the size of particles lining the
channel- increasing roughness, increases frictional
drag
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
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
Sediment size related to:
stream energy transport distance
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
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
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
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)
Deposition and Depositional features
deposition occurs when velocity decreases
competence decreases and some suspended sediments settle
out
Alluvium
unconsolidated sediment deposited by a stream
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
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
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.
Stream Valleys: Two General Types
1. narrow v-shaped 2. wide valleys with flat floors
- there is a gradation between the two types
Type of valley gives indication of the amount and
type of work the stream is doing:
v-shaped: down cutting wide/flat: lateral erosion
narrow valleys: characteristics
rapids waterfalls steep gradient; active faulting/uplift, rock
resistance above sea level
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.
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
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
Drainage patterns
give an indication about geology- dendritic- trellis- radial