water, water everywhere. “splish, splash, i was takin’ a bath…” 1.3 billion cubic km of...

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