water, water everywhere. “splish, splash, i was takin’ a bath…” 1.3 billion cubic km of...
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Water, Water Everywhere
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“Splish, Splash, I was Takin’ a Bath…”
• 1.3 billion cubic km of water at Earth’s surface• Oceans and seasAccount for ~96%Of water on Earth• Fresh water onlyAccounts for ~4%
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Recap: The Four Spheres
• Geosphere, atmosphere, hydrosphere, biosphere
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Water on Earth is ~Constant
The Hydrologic Cycle
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Following the Path…
• Water that falls on land follows one of 4 paths– Runoff—streams and rivers– Ground water—water that has seeped into the ground– Transpiration—water created by plants– Incorporated into animal and plant tissue
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StreamsStreams
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What is a stream?
• Water flowing in a channel, regardless of size
• River: – A large stream fed
by smaller tributaries
– Ex: the Nile
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How do streams form?
• Sheet flow: – Overland flow of water—usually as a thin “film”
• Irregularities in surface cause local concentrations of flow--rills
• Abrasion and positive feedback– Preferential path for water to flow, abrasion
increased
• Water can also dissolve rock
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Stream Velocity
• Factors affecting velocity– Gradient—steepness of slope—steep slope =
higher velocity– Discharge—amount of water flowing down a
stream—velocity increases as discharge increases– Channel Characteristics—shape of bed (floor) and
banks (sides)—friction between moving water and bed/banks—higher velocity in center of stream
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Discharge in more detail…• Width and depth of a stream multiplied by the
speed of the water– The volume of water that passes by a point in a
given amount of time
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Flow and Lift
• As flow gets faster and deeper:– Can move particles from pebbles to boulders
depending on speed of the flow• Occurs because movement of water over particles acts to
lift them (like a plane wing)
– Floodplains = very slow water, deposition
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Competence vs. capacity
• Capacity:– Total sediment load carried
by a flow– Increases with volume
• Competence:– A flow’s ability to carry
material of a given size– Increases with velocity
Incompetence
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Suspended Load
– Particles temporarily or permanently suspended in flow
– Size and amount increases with velocity
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How can particles be suspended?
• Gravity acts against the lift of turbulence
• Settling velocity– Depends on weight and shape
• Most particles are suspended for only a short time– Saltation
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Bed Load
– Particles carried along stream by rolling or sliding– Bed load greater particle size than suspended– Size and amount increases with velocity
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Dissolved Load– Ions dissolved in water– Dependant on discharge and chemistry– In some rivers50% of sediment load—smaller
for streams– Salty oceans
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So… What Does All of This Mean?
• The faster the current, the larger the particles carried as suspended load and bed load
• High velocity streams have the potential to carry more and larger stuff!
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Stream Development
• Downcutting• Walls
collapse and river widens
• Limited by base level
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Highland Streams (aka: youthful)
• In mountains, stream valleys are narrow, steep and there is little or no floodplain– V-shaped valley– High energy,
downcutting prominent
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Lowland Streams(aka: old streams)• In lowlands, stream
valleys are wide, gentle, and the floodplain is very wide– Low energy, mass
wasting, deposition
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Stream Development
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Low-gradient (old) Stream Features
• Low-sediment load, low velocity– Nearly flat floodplains– Meander loops
– Cut banks and point bars – Oxbow lakes
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Braided Streams
• Occur when more sediment is supplied to a stream than it can carry
• Main stream channel is filled, water spills out—new channels eroded
• Common in deserts and near glaciers
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Drainage Basins
• Major rivers fed by smaller tributaries
• Mtn ranges separate adjacent river systems
• Drainage Basin– The region drained by a
single river
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Floods• More water than
stream can hold• Spills over side of
stream onto flat flood plain
• Not always destructive– Ecosystem value– Sediment—delta– Fertile soil and flat
ground
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Floodplains + levees
• Water spills out during floods– Form natural levees– Can hold back water
even at higher levels– The danger of artificial levees
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Mississippi River
• Flood control – Levee system—prevents flood sediment from
accumulating• Increases water velocity• If breached, more severe flooding
– False sense of security– Hurricane Katrina and New Orleans
• High water + levee breach = city wide flooding– N.O. elevation at or below sea level
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