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Water, Water Everywhere
“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%
Recap: The Four Spheres
• Geosphere, atmosphere, hydrosphere, biosphere
Water on Earth is ~Constant
The Hydrologic Cycle
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
StreamsStreams
What is a stream?
• Water flowing in a channel, regardless of size
• River: – A large stream fed
by smaller tributaries
– Ex: the Nile
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
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
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
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
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
Suspended Load
– Particles temporarily or permanently suspended in flow
– Size and amount increases with velocity
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
Bed Load
– Particles carried along stream by rolling or sliding– Bed load greater particle size than suspended– Size and amount increases with velocity
Dissolved Load– Ions dissolved in water– Dependant on discharge and chemistry– In some rivers50% of sediment load—smaller
for streams– Salty oceans
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!
Stream Development
• Downcutting• Walls
collapse and river widens
• Limited by base level
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
Lowland Streams(aka: old streams)• In lowlands, stream
valleys are wide, gentle, and the floodplain is very wide– Low energy, mass
wasting, deposition
Stream Development
Low-gradient (old) Stream Features
• Low-sediment load, low velocity– Nearly flat floodplains– Meander loops
– Cut banks and point bars – Oxbow lakes
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
Drainage Basins
• Major rivers fed by smaller tributaries
• Mtn ranges separate adjacent river systems
• Drainage Basin– The region drained by a
single river
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
Floodplains + levees
• Water spills out during floods– Form natural levees– Can hold back water
even at higher levels– The danger of artificial levees
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
