Streams Nancy A. Van Wagoner Acadia University Distribution of Earth's water n What are the...

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