Streams: Transport to the Ocean Gary D. McMichael/Photo Researecher.
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Transcript of Streams: Transport to the Ocean Gary D. McMichael/Photo Researecher.
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Streams: Streams: Transport to the Transport to the
OceanOcean
Gary D. McMichael/Photo Researecher
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The Hydrologic Cycle• Describes water’s movement on the surface• P = RO + ET + I• Driven by solar heat
– ocean water evaporates – wind carries moist air over land
• topography can force moist air to higher altitudes• as air rises, it cools, water condenses to form clouds and
then precipitate as rain or snow (P)
– rain that falls on the land can:• evaporate back into the atmosphere (E)• be taken up by plants, which return water to the
atmosphere– called transpiration (T)
• flow over the ground, enter streams and rivers, and ultimately be returned to the sea; called runoff (RO)
• infiltrates and becomes groundwater (I)
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The Hydrologic Cycle
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STREAMS
This is the “RO” of
P = RO + I + ET
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StreamsStreams
StreamStream : body of water flowing in a channel
The floor of the channel is called the bedbed.
The sides of the are called the banksbanks..
FloodFlood: when bodies of water overflow their banks and water covers the adjacent land called the floodplainfloodplain.
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A stream system network.
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Drainage basinDrainage basin
Area of land surrounded by
topographic divides in which all the
water is directed to a single point
In Hawaii, divides are steep & basins are small!
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Mississippi River Drainage Basin
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Drainage Basin of the Colorado River
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Some terminology
• VELOCITY (V) - DISTANCE PER UNIT TIME (cm/s, mph)
• DISCHAGRE (Q) Total amount of water that passes a given point in a stream per unit time (m3/s) = width (m) depth (m) average velocity (m/s)
In the U.S., this is expressed as cubic feet per second (cfs): 1 m3/s = 35.9 ft3/s
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River at Low Discharge
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River at High Discharge
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Downstream changes
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Longitudinal Stream Profile of the Platt and South Platt Rivers
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FloodingFlooding
• Water in the stream is greater than the volume of
the channel.
• Interval between floods depends on the climate of
the region and the size of the channel
• In Hawaii, lots of FLASH FLOODS for mountains
are steep, flood plains are small & there is lots of
CONCRETE!
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City Built on a Floodplain
Xie Jiahua/China Features/Sygma
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Recurrence intervalRecurrence interval
AAverageverage time time between the occurrences of a given event
The recurrence intervalrecurrence interval of a flood of a given size at a given place depends on:• • climate of the regionclimate of the region• • width of the floodplainwidth of the floodplain• • size of the channelsize of the channel
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Annual Flood Frequency Curve
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More stream termsMore stream terms
competencecompetence: measure of the largest particles a stream can transport, proportional to v2
capacitycapacity: maximum quantity of sediment carried by stream, proportional to Q and v
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Job of StreamsJob of Streams
• Carry away runoffrunoff to lakes and seas
• ErodeErode land (degradation)
• Transport and depositTransport and deposit sedimentary debris
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JOB - TO ERODE
• STREAMS CUT, DEEPEN & WIDEN VALLEYS
• BY:
–HYDRAULIC ACTIONHYDRAULIC ACTION - flowing water can pick up load
–ABRASION & IMPACTABRASION & IMPACT - solid load wears down stream bed
–SOLUTIONSOLUTION - dissolves channel & load
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Waterfall Retreating Upriver
Donald Nausbaum
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Pebbles Caught in
Eddies Form
Potholes
Carr Clifton/Minden Pictures
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Stream behaviorStream behavior • Mostly determined by velocityvelocity and
shapeshape of channel.
• These factors combine to allow either laminarlaminar or turbulentturbulent flow.
• Turbulent flow is much more erosive & picks load up.
• Stream velocities may vary from 0.25 to 7 m/s0.25 to 7 m/s.
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Laminar flowLaminar flow
• Smooth sheetsheet--like flow at a low velocity
• Usually confined to edges and top of stream
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Laminar flow
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Turbulent flowTurbulent flow
• Irregular swirlingswirling flow
• Occurs at most rates of stream flow
• Keeps particles in suspensionsuspension
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Turbulent flow
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Laminar to turbulent transition
ONERA
Laminar flow Turbulent flow
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JOB - TO TRANSPORT
• Function of stream V & Q• Load is the amount of material carried
by a stream in all forms:
–Dissolved load (CATIONS)–Suspended load (fine grains)–Bed load (coarse material)
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Sediment Transport
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Saltation
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Grain Size and Flow Velocity
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Lower Velocities Form Ripples
ripple
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Higher Velocities Form Dunes
ripples
dune dune
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Giant ripples in the Channeled Scablands
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JOB - TO DEPOSIT
• As V drops, stream begins to drop its load, coarsest & heaviest first
• Forms are streams, deltas, streams, deltas, alluvial fans & flood plainsalluvial fans & flood plains
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Two important stream typesTwo important stream types
1. 1. Meandering StreamsMeandering Streams
• Gentle gradients, fine-grained alluviumalluvium
• Minimizes resistance to flow and dissipates energy as uniformly as possible (equilibrium)
• Features: point bars, oxbow lake, point bars, oxbow lake, migrating meandersmigrating meanders
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Meandering River Over Time
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Meandering River
Point Bar
Peter Kresan
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Meandering stream,Phnom Penh, Cambodia
Oxbow Lake
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Incised Meanders, Utah
Tom Bean
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Two important stream typesTwo important stream types
2.2. Braided StreamsBraided Streams
• Sediment supply greater than amount stream can support.
•At any one moment the active channels may account for only a small proportion of the area of the channel system, but essentially all is used over one season.
•Common in glacial, deserts, and mountain regions.
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Braided River
Tom Bean
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Brahmaputra River:
a braided river
Courtesy NASA
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Typical Large Marine Delta
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Alluvial Fans
Michael Collier
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Parts of a River SystemParts of a River System
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Formation of Natural Levees
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BASE LEVELBASE LEVEL
Definition is:
LOWEST LEVEL TO WHICH A STREAM CAN CUT
Think of it as:
Elevation at which a stream enters a large body of water such as a lake or ocean
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Effects of Building a DamOriginal Profile Graded to Regional Base Level
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Effects of Building a DamDam Forms New Local Base Level
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Effects of Building a DamDeposition Upstream
and Erosion Downstream
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Formation of River TerracesWhen Base Level Changes
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GEOMORPHIC CYCLE
• STAGES OF LANDSCAPE DEVELOPMENT• HILLS & VALLEYS CUT INTO SIDES OF
VOLCANOES• STAGES ARE:
– YOUTH– SUBMATURE– MATURE– OLD AGE
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GEOMORPHIC
CYCLE
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INITIAL STAGES OF STREAM EROSION IN HAWAII
• Runoff is difficult to establish for rock is very porous & slope is steep.
• Surface has to be sealed by chemical weathering or ash before it happens.
• Happens during Capping Stage• Streams tend to follow lava flow
margins or former lava rivers.• It begins at the coast & moves upwards.
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Development of an experimental drainage network - headward erosion
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Typical Drainage Networks
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Amphitheater-Headed Valleys
• In Hawai‘i, erosion of the volcanoes by stream erosion and mass wasting produces these distinctive features
• Caused by high rainfall, steep radial drainage, and alternating layers strong and weak rocks dipping seaward
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Stream Piracy orStream Capture
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planezes
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Ha‘iku Valley
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Giant “SOIL AVALANCH”
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Formation of the Nu‘uanu Pali
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Graded streamGraded stream
Stream in which neither erosionerosion nor depositiondeposition
is occurring, due to an equilibrium of
slopeslope, velocityvelocity, and dischargedischarge.
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THE END