Running WaterErosion and Weathering

The hydrologic cycle is a summary of the circulation of Earth’s water supply

Processes involved in the hydrologic cycle

PrecipitationEvaporationInfiltrationRunoffTranspiration

Evapotranspiration

Stream channel – long, narrow depression eroded by the stream into rock or sedimentStream bank – sides of channel

Stream bed – bottom of channel

During flooding, waters overflow banks onto the flood plain of valley floor

V- shaped or broad channels

Factors affecting stream erosion and deposition

Velocity – primary influenceControlled by GradientChannel shapeChannel roughness

Discharge – lesser influence

Velocity – distance water travels in a stream per unit time

Moderately fast stream – 5 km per hour (3 mi/hr)

Fast stream (flood) – 25 km per hour (15 mi/hr)Velocity is the key to stream’s ability to

erode, transport, and deposit sediments.Maximum velocity achieved in middle of

channelOutside of curve – centrifugal force – faster

velocities - erosionInside of curve – slower velocity - deposition

Stream channel – long, narrow depression eroded by the stream into rock or sedimentStream bank – sides of channel

Stream bed – bottom of channel

During flooding, waters overflow banks onto the flood plain of valley floor

V- shaped or broad channels

Factors affecting stream erosion and deposition

Velocity – primary influenceControlled by GradientChannel shapeChannel roughness

Discharge – lesser influence

Velocity – distance water travels in a stream per unit time

Moderately fast stream – 5 km per hour (3 mi/hr)

Fast stream (flood) – 25 km per hour (15 mi/hr)Velocity is the key to stream’s ability to

erode, transport, and deposit sediments.Maximum velocity achieved in middle of

channelOutside of curve – centrifugal force – faster

velocities - erosionInside of curve – slower velocity - deposition

Flood PlainsBroad strip of land built up by sedimentation on either side of a stream channel

Flooding covers land with water suspended with silt and clay, which gets deposited as flood waters recedes and slow down, flat smooth plain of sediment

Series of floods may deposit sediment in low ridges near banks creating a natural levee

Formation of natural levees by repeated flooding

Alluvial fansDevelop where a high-gradient stream leaves a narrow valley

Slopes outward in a broad arc – depositing sediment as velocity slows down

Formed in dry climates where streams don’t reach a body of water

DeltasForms when a stream enters an ocean or lake (mouth of stream)

Shape of triangle (Greek symbol delta Δ)

Consists of three types of bedsForeset beds- main body, angledTopset beds- next layer, horizontal fine-grained

Bottomset beds – in front of foreset, very fine-grained clays

Stream developmentDowncutting – process of deepening of a

valley by erosion of a stream bedLimit of downcutting – base level –

theoretical limit of erosion of earth’s surfaceV-shaped valleysSlot canyonsSlow process over timeFaster process – mass wasting – times of

floods

Lateral Erosion – widening of channel in flood plain – lateral motion

Headward Erosion – lengthening of channel by erosion at mouth

Stream terraces – step-like landforms, downcutting into new flood plains

Incised meanders – regional uplift of land

Superposed streams – erosion through the mountain range