• The alluvial deposits of Fluvisols vary between
  

1. (inland) fluvial lowlands
   a. Nature of the river systems - braided river
   b. Nature of the river systems - meandering river
   c. Position in the river system
   
2. (coastal) marine lowlands: deltas and estuaries
   

• They occur in areas with highly irregular water regime and abundant sediment supply. Often where mountains fronts enters alluvial plain.
  
• Have one single channel (may shift) of low sinuosity and a high gradient and multiple thalwegs and bars.

  Fig.1 Brahmaputra/Tibet: braided river system

  
  
• Gravel and coarse sand is deposited at the downstream ends of bars whereas the sides of bars are eroded as the thalwegs shift.
  
• Sedimentation mainly consists of coarse gravel and sand; minor inclusions of finer sediments.
  

• Have high sinuosity and the channel (river) is bordered by riverbanks or natural levees; behind occur flood basins or backswamps.

  Fig.2 The classical meandering stream

  ( Source: FAO, 2001.)

  
  
• The form is characteristic for rivers with rather steady discharge rates and sediment loads of relatively fine-grained material.
  

• Sediment depositions of fine sand and sediments take place preferentially at the inner sides of the bends (point bars) whereas gravel and coarse sand is deposited on channel floor (lag deposits).

  Fig.3 Relationship between bed-lead grain size and velocity in a section of a meandering channel: The coarsest sediment is associated with the zone of highest velocity -> outside of a bend. Fine grain-sized is associated with lowest velocity -> inside of a bend.

  
  
• During flooding: deposition of coarser sediments takes place on riverbanks

  Fig.4 Deposition of coarser sediments in riverbanks

  ( Source: FAO, 2001.)

  and clayey material in the basin.

  Fig.5 Deposition of clayey material in the basin

  ( Source: FAO, 2001.)

  
  

[Recommended webpage for further reading on sedimentation rates and form of wave and tide dominated deltas and estuaries: National Land and Water Resources Audit, Commonwealth of Australia (audit.ea.gov.au/.../ Est_Assessing.cfm) : www.nlwra.gov/au]

• 3 types of  deltas are distinguished:

  Fig.6 The Mississippi has built a series of subdeltas as it has continually dumped sediment into the Gulf of Mexico

  
  a. River-dominated deltas
  b. Wave dominated deltas
  c. Tide-dominated deltas and  estuaries
  

River-dominated deltas:

• Fluvial processes outweigh the influence of waves and tides (< 2 m tidal range) leading to a gradation of grain sizes across the delta (sorting of deposits in the delta): coarse graveled material is deposited in the delta plain close to the mouth, finer (silty) sediments accumulate in the submerged delta slope and the finest (clay) particles travel farthest to the prodelta.
  

Wave-dominated deltas (Nil, Ebro, Rhone):

• Occur in micro-tidal coastal regions where the coastal shelf has a steep gradient so that waves attain enough energy to attack and dissipate freshly deposited fluvial sediments.

  Fig.7 Wave-dominated deltas

  ( Source: audit.ea.gov.au/.../ Est_Assessing.cfm)

  
  
• Wave influenced deltas have straight outlines.
  
• Formation of river banks (fluvial deposits) and lagoons (backswamps) form in-between the beach ridges.
  
• Mangroves and channels = sedimentation areas.
  

Tide-dominated deltas and estuaries (Mekong, Ganges-Brahmaputra):

Fig.8 Tide-dominated deltas, Senegal

( Source: FAO, 2001.)

• Strong tidal influence (2 – 4 m tidal range): the fluvial sediments are re-deposited. The tidal currents widen the channels in the delta plain and sediments are deposited in opposite direction; the tidal channels have the shape of funnels with tidal flats on the overbank areas between channels.

  Fig.9 Tide-dominated Estuaries

  ( Source: audit.ea.gov.au/.../ Est_Assessing.cfm)

  
  
• Tidal flats are colonized by mangroves (sedimentation area) and add much organic debris to the fresh sediment.
  
• Major landforms are tidal channels and creeks where sand is deposited and in between them tidal mudflats with silt and clay deposition. They are strongly saline with halophytic vegetation.