module 18 - river systems

8/10/2019 Module 18 - River Systems

1/53

Module 18

Rivers System


2/53

Global Distr ibution of Water


3/53

The Hydrologic Cycle


4/53

Stream Profi les

Cross Sect ions o f Youthful and mature Streams


5/53

Drainage Basins and Divides

Drainage basinthe total area drained by a

stream and its tributaries

Dividethe high ground separating onedrainage basin from another


6/53

North American Cont inental Div ides


7/53

Drainage Basin o f the Miss iss ipp i River, Continental Divides


8/53

Stream Drainage Patterns

Form on uniformly

erodible rockThe most common

Form on mountains,

volcanoes, and domes

Form on

rectangularly

fractured rock

Form in regions of folded

shale and sandstone


9/53

Types of Sediment Loads Carr ied by Streams:

Disso lved load, Suspended load, Bed load


10/53

Stream Capaci ty and Competence

Capacity -refers to the total amount of sediment

a stream is able to transport The greater the discharge (amount of water flowing in

a stream), the greater the streams capacity

Competence- refers to the maximum size of

particles a stream can carry

The greater the velocity, the greater the competence


11/53

Base Level

A base level is a level at which no erosion can occur

A stream reaches a local base level where it flows overerosionally resistant rock

A stream also reaches a local base level where it flows into

a pond, lake, or the ocean because the gradient is 0.

Sea level is the ultimate base level

A base level can be below sea level (e.g., Death Valley)


12/53

17_14.jpgLocal Base Levels


13/53


14/53

Effect o f Channel Character ist ic s on Stream Veloc i ty


15/53



16/53

Effect o f Gradient and Channel Clutter on Stream Veloc i ty

Effect o f Channel Character ist ic s on Stream Veloc i t


17/53


Eff t f Ch l Ch t i t i St V l i t


18/53


Stream velocity is greatest in the center of the channel along straight

stretches

Stream velocity is greatest along the outside of a curve along curvedsections

R i f M i St V l i t


19/53

Regions o f Maximum Stream Veloci ty


20/53

Types of Stream Erosion

Downcutting:deepens the channel

Headward erosion: lengthens the channel

Lateral erosion:widens the channel and

flood plain

D tt i


21/53

Downcut t ing


22/53

Headward Erosion


23/53

Lateral Erosion


24/53

Lateral Erosion yields a wide floodp lain and a meander belt


25/53

Character ist ics of a Youth ful Stream

Relatively steep gradient

Narrow V-shaped valley No floodplain

Relatively straight channel

Rapids and/or waterfalls

Rocky channel filled with pebbles, cobbles,

and/or boulders


26/53

Character ist ics o f a Matu re Stream

Low gradient

Wide floodplain Meanders

Point bars

Cut banks

Natural Levees

Oxbow lakes

Meander scars

Back swamps Yazoo tributaries

Stream terraces

Y thf l t ?


27/53

Youthfu l or mature?


28/53

You thfu l or mature?


29/53

Formation of Deposi t ional and Eros ional Features

The greater a streams discharge(volume of water per

cross sectional area) the faster it is flowing

the greater its energy

the greater its capacity (amount of sediment it can carry)

the greater its competence (size of sediment grains it can carry)

Where a stream slows down, due to decreased gradient,

decreased inflow,

overflowing its banks, or

obstructions in its channel

it loses energy and therefore deposits its coarser

suspended load (sand and silt) as bars or natural levees


30/53

Formation of Deposi t ional and Eros ional Features

The suspended load

mud - a mixture of clay and silt

is deposited only under low energy conditions

such as on a lake bottom or flood plain

In Summary

Where the current is fast, energy is high, erosion occurs

Where the current is slow, energy is low, depositionoccurs


31/53

Format ion o f Point Bars and Cut Banks

A Meander Loop on the Colorado River


32/53

A Meander Loop on the Colorado River


33/53

Format ion o f Point Bars and Cut Banks

D it i f N t l L


34/53

Depos it ion o f Natural Levees


35/53

Meander Cuto ff Oxbow Lake


36/53

17_22a.jpg

Stream Terraces:


37/53

Stream Terraces:

Produc ts o f Downcutt ing , f lood ing, and lateral Erosion

Stream Terraces


38/53

Stream Terraces


39/53

A l luvial Fans

Are fan-shaped depositional features formed by

intermittently flowing streams

They form at the base of a hill or mountain where thegradient suddenly flattens

Alluvial fans commonly form where steep-gradientgullies and canyons dump into low-gradient ditches,valleys, or deserts

The sudden decrease in gradient drastically decreases

the streams energy, competence

Which, in turn, causes the stream to drop its

bed load nearest the hill

suspended load farthest away, in the toe of the fan

A lluv ial Fan in Death Valley


40/53

17_21a.jpgA lluv ial Fan in Death Valley


41/53

Braided Streams

Are continuously-flowing, sediment-overloaded streams

that flow in networks of interconnected rivulets aroundnumerous channel bars

They form where steep-gradient gullies and canyons

dump into low-gradient ditches, valleys, or plains

They also form down hill of melting glaciers, in glacial

valleys and on their outwash plains

The sudden decrease in gradient drastically decreasesthe streams energy, competence

Which, in turn, causes the stream to become sediment-

overloaded and drop its bed load and suspended load

as channel bars


42/53

17_21b.jpg

Braided Stream

Channel bars

Deltas


43/53

Deltas

Are fan-shaped depositional features formed at the

mouth of a stream where it flows into a large body of

relatively still water, such as a lake or ocean

Thus, the stream has reached a base level, where the

stream suddenly loses energy and competence

because the gradient is flat (horizontal) The stream diverges into small, shifting channels,

distributaries, that carry sediment away from the main

channel and distribute it over the surface of the delta

The topset beds and bottomset beds deposited in a

delta are subhorizontal

The foreset beds, deposited where the water suddenly

deepens, dip shallowly to steeply seaward

F t f D lt


44/53

Features o f a Delta


45/53


46/53

17_24c.jpgThe Miss iss ipp i Delta

Evolut ion of the Miss iss ipp i Del ta


47/53

17_25a.jpgEvolut ion of the Miss iss ipp i Del ta


48/53

Floods

Floods are the most common and most

destructive geologic hazard

Floods result from naturally occurring and

human-induced factors

Causes of flooding include

heavy rains

rapid snow melt dam failure

topography

surface conditions

B ig Thompson Canyon Flash Flood (1976) and


49/53

Fort Co ll ins Flash Flood (1997)

Devastat ion caused by th e Big Thompso n Canyon Flash Flood , 1976


50/53

Devastat ion caused by the Fort Col l ins Flash Flood, 1997


51/53


52/53

17_34a.jpg


53/53

Flood Contro l

Engineering efforts include

building artificial levees

building flood-control dams

clearing and straightening channels

Good floodplain management

module 18 - river systems

Documents