Page 353 - Fundamentals of Geomorphology

336 PROCESS AND FORM

Figure 13.15 Factors affecting dune morphology. For explanation, see text.
Source: Adapted from Pye (1990)

tide levels. As well as major tidal channels, tidal creeks to form when tidal flats are high enough to permit col-
flow across tidal flats.These are shallower than tidal chan- onization by salt-tolerant terrestrial plants. Depending
nelsandrundowntolow-tidelevel.Onmuddytidalflats, on their degree of exposure, salt marshes stretch from
tidal creeks often display a dendritic pattern with wind- around the mean high-water, neap-tide level to a point
ing courses and point bars. On sandy tidal flats, tidal between the mean and extreme high-water, spring-tide
creeks have ill-defined banks, straight courses, and few levels. Their seaward edge abuts bare intertidal flats, and
tributaries. their landward edge sits where salt-tolerant plants fail
Tidal flats are built up from clay-sized and fine silt- to compete with terrestrial plants. Salt marsh sediments
sized sediments carried to the coast by rivers. On meeting are typically heavy or sandy clay, silty sand, or silty peat.
salt water, particles of clay and silt flocculate (form clot- Many salt marshes contain numerous shallow depres-
likeclusters)tobecomelargeraggregates.Theythensettle sions, or pans, that are devoid of vegetation and fill with
out as mud in quiet coastal waters such as lagoons and water at high spring tides.
sheltered estuaries.The mud is carried in by the incoming
tide and deposited before the tide reverses. If the mud
continues to build upwards, a part of the tidal flat will
be exposed just above normal high-tide level. This area Mangals
is then open to colonization by salt-tolerant plants, and ‘Mangrove’ is a general term for a variety of mainly
salt marshes or mangroves may develop. tropical and subtropical salt-tolerant trees and shrubs
inhabiting low inter-tidal areas. Mangals are communi-
Salt marshes ties of mangroves – shrubs and long-lived trees and with
associated lianas, palms, and ferns – that colonize tidal
Salt marshesarewidespreadintemperateregions,butare flats in the tropics, and occur in river-dominated, tide-
not uncommon in the tropics (Figure 13.19). They start dominated, and wave-dominated coastal environments