Geoscience Reference
In-Depth Information
Mississippi
Figure 17.13
Classification of river deltas
fluvial, wave and tidal
processes.
Source: After Galloway (1975)
Fluvial processes
Modem
Mississippi
Po
Danube
Danube
Fluvial
Dominated
Yukon
Mahakam
Mahakam
Ebro
Nile
Orinoco
Wave
Dominated
Tide
Dominated
Niger
Mekong
Yalu
Colorado
Rhône
Kelantan
Brazos
San Francisco
Nile
Fly
Ganges-Brahmaputra
Kiang-Langat
Copper
Wave processes
Tidal processes
Copper
San
Francisco
Fly
(a)
Top-set beds
mixing. Freshwater and denser salt water override and
undercut each other respectively in river-dominated
estuaries. The inland extent and slope of the
saltwater
wedge
is determined by the opposing vigour of river
discharge. Its shifting boundary marks a concentrated
zone of clay-silt particles where
flocculation
into larger
aggregates encourages them to settle as mud. Tidal mixing
produces vertical homogeneity, with salinity increasing
steadily seawards. Open lagoons connected to the sea by
tidal passes normally maintain balanced tidal exchanges,
except in arid-zone
sabkhas
, where strong evaporation
precipitates salts and reduces return flow. Closed lagoons
are rain-fed and, whilst storm washover, lateral seepage
through permeable barriers and evaporation may restore
some salinity, it varies between freshwater-brackish-
hypersaline extremes. Narrow
tidal passes
through
barriers connect lagoons, and estuaries with restricted
mouths, with the sea. They create their own distinct water
and sediment fluxes to form microcosms of full-scale
coastal landsystems (
Figure 17.15
).
Fore-set beds
Bed load
Bottom-set beds
(b)
Bed load
(c)
Bed load
Flocculation and rainout,
suspended load
Water body
Sediment
Sediment movement
lacustrine basins. Gilbert-type delta formation (a) by rapid
deposition of bed load contrasts with sediment plumes
entering less-dense (b) or denser water (c).
Source: Partly after Elliot (1986)
