FLUVIAL LANDFORMS

A. Introduction

• lateral migration, flooding, erosion & deposition affect channel pattern

• also result in other landforms: floodplains, fluvial terraces and alluvial fans

B. Floodplains

1. Definition

a. topographic

b. hydrologic

2. Floodplain landforms

3. Floodplain deposits

a. lateral accretion

• point bar deposits

b. vertical accretion

• overbank sediments

• natural levees

• splay deposits

c. collluvium

d. channel deposits

• transitory

• lag

• fill

4. Floodplain formation

a. lateral erosion & deposition

b. overbank flow

c. exceptions: arid climates & braided channels

C. Fluvial Terraces

1. Definition

a. abandoned floodplains

• indicate prolonged episodes of degradation

• terrace tread, terrace scarp

b. classification

• genetic: erosional, depositional

• topographic: paired, unpaired

2. Erosional terraces

a. tread formed by lateral erosion & deposition

b. point bars become capping alluvium

c. underlying surface mirrors tread

3. Depositional terraces

a. tread formed by uneroded surface of valley fill

b. capping alluvium of variable thickness

• thickness greater than scouring depth of river

c. underlying surface not mirror image of tread

4. Distinction of erosional terraces from depositional terraces

D. Alluvial Fans

1. Definition

• depositional feature found where mountains grade into plains

• appears as a cone segment radiating away from a point source

• may coalesce with adjacent fans forming a bajada

2. Fan components

• feeder channel

• fan apex

• incised channels

• intersection point

• abandoned channels & discontinuous gullies

3. Fan deposits & origins

• deposition initiated by change in hydraulic geometry

• nature of deposits nature of flow, in particular, water/sediment ratio

• fan history difficult to interpret

E. Summary

• fluvial landforms include: stream channels, floodplains, fluvial terraces & alluvial fans

• all result due to:

  • interaction between driving forces (climate & gravity) & resisting forces (lithology & geologic structure)

  • hydraulic conditions & sediment load

  • processes of erosion, transportation & deposition