RiversSculpting Earth’s Surface

Rivers of the World

• Not a large proportion of Earth’s water

• A major contributor in sculpting the landscape

Types of Flow

Laminar Flow– Low velocity– Particles flow in

straight lines– No mixing– Rare in water systems

Turbulent Flow– High velocity– Particles move in

irregular paths– Mixing– Most common type of

flow

Flows in Rivers

• Velocity

– Fastest in center; slowest at edges

– discharge

– distance per unit of time (ft/s or m/s)

– gradient

• Factors in velocity

– size & roughness

Gradient

• Gradient (slope)

– vertical drop over distance– highest @ headwater; lowest near base

Channel Characteristics

•Size

•Roughness

– larger channels have more efficient flow

– Smooth channels promote more uniform flow– Irregular channels

create turbulent flow

Discharge

• Volume of water flowing past a certain point in a given unit of time (m3/s)

Erosion, Transportation and Deposition

• Rivers typically display three different behaviors

Erosion – breakdown and removal of material

Transportation – movement of material down a riverDeposition – accumulation along banks and beds of rivers

Anatomy of a River

• Headwater

• Plains

• Delta

– high slope area– erosion dominant

– transportation dominant

– gradient decreases

–Base level–Deposition dominant

X-section of a River

• Features– Constantly decreasing

gradient– Overall, smooth and concave upward

curve

Erosion

•Particles are dislodged from channel and lifted•The stronger the flow, the more particles•More particles, increase erosion (sandpaper)

Transportation

• Bed load – moves along bottom by skipping or sliding

• Suspended load – fine sediment carried above channel – silt and mud

• Dissolved load – sediment carried in solution - salt

Transportation• Criteria of a stream’s ability to carry a load

• Competence

• Capacity

–Measures maximum size

– Related to velocity

–Maximum load

– Related to discharge

Transportation - Landforms

– Complex stream channels separated by sand bars

– Curving stream that migrates side-to-side• Meandering Stream

• Oxbow Lake

• Braided Stream

– A curved lake produced when a stream cuts off a meander

– Coarse material; bedload

– Fine grained; suspended load

Landform Formation

• Cutbank – banks that are eroded

• Point Bar – areas of deposition

Examples of erosion and deposition:

Fluvial Landforms Examples

Braided

Meandering

Oxbow

Deposition

• Base level – limit to where a stream can no longer erode

• When velocity approaches 0 m/s, deposition occurs

− Sea level – ultimate base level

− Lakes and dams local base level

Drainage Basins• Drainage Basin – The surface area that contains

tributaries and supplies the surface runoff that feeds the river

• Dendritic – irregular branching; uniform underlying material• Radial – diverge from a central area; volcanic domes• Rectangular – right angle bends; contains joints and faults• Trellis – parallel; alternating bedrock

Floods

•Overflow of a stream channel when discharge exceeds channel capacity

•The most common and destructive geologic hazard

New Orleans, 2005

Floods

• Causes of Flood– Major storms– Rapid snow melt– Hurricane

Pittsburgh 1996

Sepulveda Basin

Flood Control

•Artificial Levees • Channelization