streams part 1

8/3/2019 Streams Part 1

1/50

Streams and Drainage Systems


2/50

Streams in the Landscapel A stream is a body of water that:

Flows downslope along a clearly defined natural passageway. Transports detrital particles and dissolved substances.

l Channel The passageway of a stream.

l Load The sediment and dissolved matter the stream transports.

l Discharge

The quantity of water passing by a point on the stream bankin a given interval of time.


3/50

Human Use of Streamsl Large streams are important avenues of transportation.

l People choose to live near streams b/c: Valley floors are flat and easy to build on. Soils tend to be deep and fertile. Water is available.

l Stream valleys have drawbacks. They are threatened by floods. Human and industrial wastes pollute the water.


4/50

Streams as Geological Agentsl Streams are vital geologic agents.

Carry most of the water that goes from land to sea

(essential part of the hydrologic cycle).

Transport billions of tons of sediment to the oceans

each year.


5/50

Stream Channelsl A streams discharge may vary due to changes in

Precipitation Melting of winter snow cover.

l

Geometry of stream channels is not constant. Shape and orientation of channel change with varying

discharge and load.

l Two ways to characterize a channel are: By measuring its cross-sectional shape. By determining its long profile.


6/50

Cross-Sectional Shapel The volume of water moving through a channel

generally increases downstream.

The ratio of channel width to channel depth is

likely to change down stream.


7/50

Long Profilel The gradient of a stream

The difference in elevation over a known distance along itscourse.

Decreases downstream.

l The streams long profileis a line drawn along thesurface of a stream from its source to its mouth. It is a curve that decreases in gradient downstream.


8/50

l A long profile is NOT a perfectly smooth curve b/c

irregularities in the gradient can be produced by:l Beds of resistant rock.l Landslides.l Lava flows.l

Hydroelectric dams.

Long Profile


9/50

Dynamics of Streamflowl The average annual rainfall in the U.S. is equivalent to

a layer of water 76 cm thick. 45 cm returns to the atmosphere by evaporation and

transpiration.

1 cm infiltrates the ground, The remaining 30 cm forms runoff

(the portion of precipitation that

flows over the land surface).


10/50

l Initially water tends to move down slopes in broad,

thin sheets. This process is called overland flow.

l After traveling a short distance, overland flow begins

to concentrate into well-defined channels calledstreamflow.

Dynamics of Streamflow


11/50


12/50

Changes Downstreaml Traveling down a typical stream from its head to its

mouth: Discharge increases.

Stream cross-sectional area increases.

Velocity increases slightly.

Gradient decreases.


13/50

l

On steep mountain slopes, discharge is low so theflowing water is shallow. The stream bed causes much more resistance to the

flow of shallow water.

l

Discharge increases downstream as each tributary (astream joining a larger stream) and inflow ofgroundwater introduce more water.

l To accommodate the greater volume of water, velocity

increases together with the cross-sectional area ofthe stream.

Changes Downstream


14/50

Floodsl Flood occurs when

A streams discharge > the capacity of the channel. A stream overflows its banks.

l Increased discharge Increase in the water level and erosion scouring the bed.

Changes in cross sectional area of Colorado River

l Increase in velocity & the enlarged channel

Accommodate the increased flood discharge & a greaterload.

l Decrease in discharge after flood The stream is unable to transport as much sediments.

Stream channel returns to its pre-flood dimensions.


15/50

l Unusually large discharges associated with floods

appear as major peaks on a hydrograph.l During a flood

Discharge increases Increase in flow velocity. Velocity increase enables stream to carry:

l Greater load.l Larger particles.

Floods


16/50

Flood predictionl A flood-frequency curve

Plot of the occurrence of past floods of different sizes.

l Recurrence interval The measure of how often a flood of a given magnitude is

likely to occur.l Global climate change

Present flood-frequency curves may be a little value in pre-dicting future floods.


17/50

Catastrophic Floods: An Example

l A huge glacier-blocked lake in Montana during the last

glacial age. The lake contained 2,000 ~ 2,500 km3 of water. Remained in existence as long as the ice dam was stable.

l Retreat of the glacier

The dam failed and waterwas released rapidly.

Analogy to a plug pulled from

a giant bathtub.

l The main exit route Across the channeled Scabland

region and down the Columbia

River to the sea.


18/50


19/50

Base Levell Potential energy of water

Decreases as a stream flows downslope Falls to zero as it reaches the sea.

l Base level The limiting level below which a stream cannot erode

the land. Global sea level for most.


20/50

l Exceptions Streams that drain into closed interior basins having no

outlet to the sea. Where the floor of a tectonically formed basin lies below

sea level, the base level coincides with the basin floor.

l Death valley, California When a stream flows into a lake, the surface of the lake

acts as a local base level.

Base Level


21/50

Natural and Artificial Damsl Natural dams

Landslide sediments. Glacial deposits. Glacier ice. Lava flows.

Acts as a local base level and creates an irregularity ina streams long profile.l Large artificial dams. Being constructed in ever-increasing numbers for:

l Water storage.l Flood control.l Hydroelectric power.


22/50

l Hydroelectric power

Potential energy change in streams as they flow downslopeto the sea.

A renewable resource.

l An artificial dam & reservoir

The reservoir traps nearly all the sediment that the streamformerly carried to the ocean.

Reservoir siltation limits the useful life of power dams.

Natural and Artificial Dams


23/50

Channel Patternsl Straight channels are rare.

The highest velocity along a straight channell Near the surface in mid-channel.

l If a stream channel has many curves the channel

pattern is sinuous.

l A deposit of sediment (a bar) tends to accumulatewhere velocity is lower.

l In many stream, the channel forms a series of

meanders.


24/50

Meandersl Occurrence

Fine-grained stream sediments and gentle gradients.

l Velocity Lowest where there is maximum frictional resistance to flow.

l Migrate slowly down a valley w/ time.l The inner side of each loop

Water is shallow and slow. Coarse sediments form a point bar.


25/50

l When the next meander upstream migrates more

rapidly, it may intersect the slower moving meander. Meanders are cut off.

The cut off meander is converted into a curved oxbow lake.

Meanders


26/50


27/50

Erosion by Running Water

l Erosion

Process of weathering and transport of solids.

Begins before formation of a distinct stream.l By impact of raindrops hitting the ground.

l

By overland flow during heavy rains (sheet erosion).l Effectiveness of raindrop and overland flow erosion.

1/vegetation

Vegetation provides a protective cover.


28/50

l Mode of water movement The ability of streams to erode is influenced by the way water

moves through a stream channel.

l Laminar flow The velocity of water flow is very slow. The water particles travel in parallel layers.

l Turbulent flow

With increasing velocity, the movement becomes chaotic.

l The ability of a stream to pick up and move particles of

sediment from its channel

depends largely on:

Erosion by Running Water


29/50

The Streams Loadl The load of a stream

Solid load: Those that are not dissolved. Dissolved load: Product of chemical weathering.

l The solid portion: Bed load

l The coarse particles that move along the stream bed. Suspended load

l Fine particles that are suspended in the water.l Alluvium

Loose, unconsolidated soil or sediments eroded, transported

and deposited by nonmarine water. The solid portion constitute alluvium wherever they are

dropped.


30/50

Bed Loadl Constitutes 5 ~ 50 % of the total load of a stream.

l Particle movement Discontinuous movement by rolling or sliding Velocity of movement slower than the stream water.

l Salatation Series of short intermittent jumps.

l Sediment grain size @ the sediment bed Flow velocity


31/50

l Sediment deposits in

the meandering stream


32/50


33/50

l Most placer gold occurs as grains the size of silt

particles, the gold-dust of miners.

l Nugget: larger gold placers.

l Can also be found w/ Pt, Cu, Sn, diamond, ruby, andsapphire.

Placer Deposits

Gold dust

Nuggets

d d d


34/50

Suspended Loadl Fine particles of silt and clay moving in suspension

make the stream look muddy.l How do they keep suspended?

Upward-moving currents within a turbulent stream> The velocity at which particles can settle toward the bed

under the pull of gravity.l Deposition

Only where velocity , and turbulence ceasesl in a lake or in the sea.

D l d d


35/50

Dissolved Load

l Water

An excellent solvent. All stream water contains dissolved chemical substances.

l The bulk of the dissolved content of most riversconsists of seven ionic species: Bicarbonate (HCO3-). Calcium (Ca2+).

Sulfate (SO42-). Chloride (Cl-). Sodium (Na+). Magnesium (Mg2+).

Potassium (K+).

D h P l


36/50

Downstream Changes in Particle Size

l Decreases in coarseness downstream.

In mountainous headwaters, rivers may transportlarge boulders.

l Temporal change Coarse bed load gradually gets smaller by abrasion. Bed load @ land-sea boundary

l May consist mainly of sediment no coarser than sand.

Sediment sizes in the Mississippi River

D Ch C


37/50

Downstream Changes in Compositionl The Nile includes three major tributaries:

White Nile.l 1/3 of total discharge.

l 3% of bed load

Blue Nile.l Drians highland Ethiopia

l >1/2 of discharge

l ~75% of bed load

Atbara.l 14% discharge

l ~25% of bed load

S di Yi ld


38/50

Sediment Yieldl Sediment yield

The amount of sediment/unit area eroded from theland surface and delivered to a stream system.

A function of:l Rock type and structure.

l Local climate.l Relief and slope.

Precipitation. Plant in moist regions

l Plant roots tend to anchor the soil, thereby curtailingerosion.

l Low erosion rate in temperate eastern North Americaand Western Europe due to continuous vegetation cover.

di Yi ld


39/50

l Higher erosion rates in drier regions, Limited vegetation due to low precipitation.

l The greatest local sediment yields observed from desert area.

l Some of the highest measured sediment yields Basins that drain steep mountains along plate boundaries. Monsoon regions of southeastern Asia.

l High runoff from abundant precipitation .

Sediment Yield


40/50

St D it


41/50

Stream Depositsl Decrease in turbulence

Drop in transporting power & deposition of part of stream load.

l Major flood The water overflows the banks & inundates the adjacent floodplain. Natural levee

l The boundary between channel and floodplain

l a broad, low ridge of alluvium built along the side of a channel bydebris-laden floodwater.

l Terrace Relatively flat alluvial area that lie above the floodplain.

A terrace is a remnant of an abandoned floodplain.

All i l F


42/50

Alluvial Fanl Changes in conditions as the stream moves

A steep upland valley a nearly level valley floor or an alluvial plain,

l Decrease in slope,l Corresponding drop in velocityl

Decrease in its ability to carry sediment.l Formation of a fan-shaped sedimentary structure

Alluvial fan.

D lt


43/50

Deltasl Similar formation mechanism to what forms alluvial

fan. A stream enters the standing water of the sea or a lake.

l Fan delta Typically built adjacent to a mountain front.

l Braid delta A coarse-grained delta constructed by a braided stream.

D in S t m


44/50

Drainage Systemsl Drainage basin

Total area that contributes water to the stream.

l Divide The line that separates adjacent drainage basins.

l Stream order The arrangement and dimensions of streams in a drainage

basin tend to be orderly. 1st-order streams

l The smallest segments that lack tributaries.

D i S t


45/50

2nd-order streamsl Formed where two first-order streams join.

3rd-order streamsl Formed by the joining of two second-order streams.

l Stream capture The interception and diversion of one stream by another

stream.

Drainage Systems

E l ti f D i


46/50

Evolution of Drainagel Upstream development

Early development of stream in the region close todrainage area.

Erosional progression in the upstream area. Acquisition of tributaries from stream capture.

Drainage Patterns


47/50

Drainage Patterns

D i S t & G l


48/50

Drainage System & Geology

Tampering with the Nile


49/50

Tampering with the Nilel Prior to construction of the Aswan Dam, an average

of 125 million tons of sediment passed downstreameach year. The dam reduced this value to only 2.5 million tons. Nearly 98 percent of the suspended sediment is now

deposited in the reservoir behind the dam.

T mp i ith th Nil


50/50

l Under natural conditions this sediment was carried

downstream by floodwater, where much of it was

deposited over the floodplain and delta, thus adding to

the rich agricultural soils at a rate of 6 to 15

cm/century.

l Because the annual discharge of sediment has nowbeen cut off, the coast has become increasingly

vulnerable to erosion.

Tampering with the Nile

streams part 1

Documents