Water, Water Everywhere

“Splish, Splash, I was Takin’ a Bath…” 1.3 billion cubic km of water at Earth’s surface Oceans and seas Account for ~96% Of water on Earth Fresh water only Accounts for ~4%

Recap: The Four Spheres Geosphere, atmosphere, hydrosphere, biosphere

Water on Earth is ~Constant The Hydrologic Cycle Water on Earth is ~Constant

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

Streams

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 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 rivers50% 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