Geography/Geology 352: Geomorphology

FLUVIAL LANDFORMS

A. Introduction

• Interaction of:

  • driving force of water &

  • resisting force of landscape

  • results in numerous fluvial landforms at a variety of scales (spatial & temporal

• Fluvial system works to maintain equilibrium:

  • as thresholds are crossed, fluvial landscapes undergo change (temporary or permanant)

• Agenda - examine:

  • stream channel patterns, their formation & associated landforms

  • floodplain formation & characteristics

  • fluvial terrace formation

  • graded profiles (knickpoints)

photo: T.A. Blake. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

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B. Single Strand Channels

1. Straight versus meandering

• sinuosity: stream channel length divided by length of meander belt axis or by valley length

photo: S. Hillebrand. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

• sinuosity of 1.5 is dividing point between straight & meandering

  photo: A. Copley © Oklahoma University. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ID hn86ji

  photo: S. Hillebrand. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

2. Characteristics meandering thalweg alternate point bars, pools, riffles, & cutbanks meander growth & downstream migration bar and swale topography variable cross-channel profile

photo: M. LeFever. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

Alaska photo: U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

 

3. Pools and riffles

a. characteristics at low discharges

RIFFLES	POOLS
shallow (& wide)	deep (& narrow)
high velocity	low velocity
steep water surface gradient	gentle water surface gradient
coarse grained bed material	fine grained bed material
divergent flow (facilitates deposition)	convergent flow (facilitates scour)

b. secondary flow patterns: spiral or helical flow

• outside of bends - water surface is elevated, which generates an accelerating downward motion, which scours the bank

• inside of bends - flow is decelerating and moving upwards, which results in deposition of material

c. formation

• velocity reversal hypothesis: pools & riffles form when discharge exceeds a threshold such that pool velocity exceeds riffle velocity

• as discharge increases, rate of change in:

  • depth is greater in riffles than in pools

  • velocity is less in riffles than in pools

  • water surface slope is less in riffles than in pools

  • result: as flow increases, pools & riffles become more similar

• above a threshold discharge, critical bed velocity & bed shear stress are greater in pools than in riffles

 

4. Meanders a. meander development & growth meandering thalweg alone is insufficient to generate a meandering channel requires local, rather than widespread bank erosion requires lateral erosion & deposition requires helical flow transporting material from meander bend and depositing it in riffle or next point bar downstream b. meander cutoffs oxbow lakes meander scars c. balance between available energy & available sediment (Boyce, LA 1970, 1981, 1994 & 2004)

Wisconsin photo: © Louis Maher. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ID hysfdg Alaska photo: S. Hillebrand. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/

5. Step-pool sequences

• gravel bed rivers with gradients steeper than 2-3%

• steps: accumulations of woody debris, bedrock or interlocking cobbles & boulders; high gradient & velocity

• pools: fine grained bed material; low gradient & slow flow

 

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C. Multi-strand Channels

1. Braided water strands divide around coarse-grained, unstable bars frequent changes in size, location & number of bars total channel width is large compared to channel depth gradient generally steeper than meandering rivers Factors associated with braiding: easily eroded banks - widespread bank erosion abundant bed load rapid & frequent variations in discharge disallows vegetation to establish on bars

photo: M. Emery. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/ photo: K.A. Lemke

2. Anabranching relatively permanent system of multiple sinuous channels with cohesive banks Formation avulsion: local occurrence of overbank flow cuts new channel into existing floodplain deposition results in formation of enchannel ridge that diverts flow into 2 directions both processes promoted by: stable, cohesive banks that limit channel widening one or more mechanisms that promote localized overbank flooding

photo: H.J.A. Berendsen http://www.geo.uu.nl/fg/palaeogeography/results/fluvialstyle photo: © Michael Collier. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ixvt9i

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D. Floodplains

1. Topographic and hydrologic floodplains (photo: Red River floodplain 1995, NASA http://earth.jsc.nasa.gov/sseop/efs/lores.pl?PHOTO=STS073-726-31) 2. Formation a. lateral erosion & deposition b. overbank flow 3. Exceptions

photo: A. Booher/FEMA http://www.photolibrary.fema.gov/photolibrary/index.jsp photo ID: 3459

4. Deposits

a. lateral accretion

• point bar deposits

b. vertical accretion

• overbank sediments

• natural levees

• splay deposits

c. channel deposits

• transitory

• lag

• fill

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E. Fluvial Terraces

1. Indicate prolonged episodes of degradation tectonic uplift base level drop

photo: © Michael Collier. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ivq0ux

2. Formation

a. erosional

• tread formed by lateral erosion & deposition

• point bars become capping alluvium; uniform thickness

b. depositional

• tread formed by uneroded surface of valley fill

• capping alluvium of variable thickness

  

 

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F. Graded Profile

1.

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G. Summary

1. Fluvial landforms result from interaction of: driving forces (water flow; provides energy to do work) resisting forces (lithology and characteristics of channel bed & banks) processes of entrainment, transportation, & deposition of sediment (types of work done) 2. System equilibrium maintained by: erosion when excess energy is available - degradation or lateral migration deposition when insufficient energy is available - aggradation or lateral deposition available energy & sediment change over time and space, thus erosion & deposition change over time and space 3. Ongoing work to maintian equilibruim results in various landforms single strand and multi-strand channels and associated characteristics floodplains fluvial terraces 4. Thresholds are crossed when a change occurs system may return to previou equilibrium system may establish new equilibrium result: fluvial landforms change over time and space

photo: © B. Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ID h27o4h photo: R. Hagerty. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/