Fluvial River Environments

1
Fluvial (River) Environments

• The specification states that you must be able
  to
• Describe the characteristic products of
  sedimentation in fluvial environments
• alluvial fan breccias, arkoses and conglomerates
• channel sandstones
• flood plain clays and silts
• and explain the processes which formed them.

2
Alluvial Fans

• These are fan-shaped deposits formed at the base
  of mountains.
• Alluvial fans are most common in arid and
  semi-arid regions where rainfall is infrequent
  but torrential, and erosion is rapid.
• The sediment is typically coarse, poorly- sorted
  gravel (conglomerates and breccias) and sand
  arkose).
• the energy can be high (flash floods) and the
  sediment is deposited rapidly.

3
River Channels

• The channels can be braided and meandering.
• Channel deposits consist of coarse, rounded
  gravel (conglomerates), and sand (symmetrically
  rippled sandstones).

• The channel deposits are channel (lens) shaped.
• Point bars are made of sand or gravel.
• Levees (raised banks) are made of fine sand or
  silt.

4
Floodplains

• Floodplains are covered by silt and clay.
• Sediments may be organic and contain plant
  fossils

5
Sediment distribution along a river

• It will start off coarse and poorly sorted and
  get gradually finer and better sorted.
• This reflects the decrease in energy and more
  constant rates of deposition

6
River channels

• Channels move over time creating many different
  channels
• These channels on the southern side of the
  Himalayas have moved east to west in 230 miles.
• Channels show both lateral and vertically
  variation

7
River channels
Migrating channels leave cut off meanders (oxbow
lakes)
Point bar deposits move laterally as channel
erodes and migrates
8
Fining up graphic log

• Base of river channel marked by gravel
  conglomerate
• Point bar marked by cross bedded sand
• Floodplain clay and silt

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