Landforms of the Fluvial System
Objectives
• Describe formation and characteristics of alluvial fans
• Examine landforms formed by fluvial erosion and deposition in river valleys
• Investigate the evolution of river deltas
Introduction
• Fluvial landforms built by:– Deposition– Erosion
Alluvial Fans• Ephemeral streams
emptying from mountain valleys construct alluvial fans
• Alluvial fans are fan-shaped– Formed of alluvial
material– Debris flows in upper
alluvial fans• More common in deserts– But can form in humid
areas
Flow and Deposits on Alluvial Fans
• Flow emptying from valley spread– Velocity decreases,
sedimentation occurs– Unconfined water spreads across
fan in shallow channel
• Deposits are poorly stratified– Thick near mountain– Thin outward from mountains
• Alluvial fans coalesce to form bajadas
• Older parts of fan may develop desert varnish/pavement
Landforms of River Valleys-Meandering Rivers
• Bends in sinuous channel termed meanders– Erosion on outside of
bend– Deposition on inside of
bend—Pt bar– Grow and migrate– Cut-off meanders form
oxbow lakes
Landforms of River Valleys-Meandering Rivers
• Formation of oxbow lakes
Landforms of River Valleys - Braided Rivers
• River with multiple channels separated by sand or gravel bars
• Braiding forms by deposition of bar in mid-channel– Water flows around bar
forming two channels• High sediment load, steep
slopes, variable discharge, loose bank materials may cause braiding
Landforms of River Valleys - Floodplain
• Flat, low-lying ground bordering channel
• Frequently flooded and sediment deposited– Levees form on channel
margin
• Eroded by migrating meandering
• Growing pt. bar rebuilds floodplain
Landforms of River Valleys - Floodplain
• Widening of floodplain and devleopment of large meanders in meander belts
Landforms of River Valleys - Floodplain
• Flooding and levee development
Human Use of Floodplains
• Farming• Development• People do not adjust
well as risk grows• Flood control measures– Artificial levees, dams– Provide mostly artificial
security– Must assume floods will
recur
Terraces• Benches above modern
floodplain– Are older floodplains
• Develop through stream incision of floodplain– Lower floodplain develops– May have paired or mostly
unpaired terraces• May have aggradation before
terrace formation– Increased sediment supply
causes– Reduced sediment supply
then results in incision– Fill terraces produced
Terraces
• Aggradation and terrace development
Deltas
• Triangular shaped deposits where streams empty into oceans or lakes– Coarse sediments deposited at river mouth– Fine sediments carried farther– Distributary channels form transporting water and
sediments
• Delta form depends on:– Discharge and sediment transported– Configuration of seafloor– Strength of currents, waves, tides
Delta Types
The Delta Profile• Consists of bottomset, foreset, and topset beds• Compaction may occur causing subsidence
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