• built from primarily river-borne sediment
• form when the amount of sediment delivered at the mouth of a river exceeds the amount removed by waves and tidal currents
• resemble estuarine bays

where do deltas occur?

trailing edge and marginal sea coasts provide the right conditions for delta formation

• rivers bring an abundant sediment supply across wide, gently sloping land
• broad continental shelf provides a platform for sediment accumulation
• not all trailing edge coasts develop major deltas

how old are the deltas?

most are very young geological features - some only a few 100s years old

young deltas
controlled in part by sea level...

older deltas
some rivers have built their deltas on top of older deltas built up during previous interglacial periods; e.g. Mississippi delta is underlain by deltas 10s of M years old forming different lobes (new lobes are formed whenever the location of the river mouth changes); present lobe of the Mississippi delta is 600 years old
 

environments of the delta

depends on the interaction between the river's sediment flow and distribution and the wave and tidal currents; as water flows from the river's mouth, its velocity decreases and it loses its capacity to carry sediment - sediments accumulate in the river mouth area and the river divides into distributary channels

Huang (yellow river), China: river with the greatest sediment load

delta plain: landward and very flat part of the delta

natural levee: when a distributary channel overflows its banks, the coarser sandy sediment particles are dumped first, producing a low ridge along the bank; height is 1-2 m

crevasse splay: a thin fan shaped sediment accumulation that occurs at a breach (crevasse) in the natural levee during subsequent flooding

marsh grasses: further toward the ocean, gradient of land flattens and height of natural levees decrease

salt marshes: in warm climates, mangrove swamps flourish; brackish water

delta front: where wave and tidal energies affect coastal development; fine sediment is carried away from the distributary mouth by wave and tidal currents, whereas sandy sediment is deposited at the channel mouth

channel mouth bar: sand accumulation at a distributary mouth... causes the channel to bifurcate

floodwaters:

• upper delta plain: sediment contributes to floodplain deposition
• lower delta plain: sediment settles in distributary bays and marshes eventually filling them and becoming vegetation-filled marshes

river vs. sea

depends on rainfall and soil type

erosion of the delta: prolonged draught will allow erosion of a delta

• strong tidal currents
  
• energetic waves
  
• human impact
  

• dominant river flow
  

delta shapes

triangle model - William Galloway, University of Texas classification scheme

• river, wave, and tide dominate at apexes

• ample flow of freshwater and sediment from the upland and a placid seaward receiving basin

• freshwater discharge is overpowered by tidal current

• typically smooth shoreline 
• longshore currents carries sediment away
• two types:
• symmetric
• asymmetric

intermediate delta types

e.g. Nahakam River: river and tide influenced delta

e.g. Nile River: river and wave influenced delta

e.g. Niger River: river, wave, and tidal influenced delta
 

human intervention

development to take advantage of fertile soil and nearby fisheries

enhance delta growth

reduce delta size

attempts to control deltas has weakened them...  almost impossible to return them to a more natural state.