24. ESTUARIES, DELTAS, SHELF, SLOPE 



A. Gfilcher 



1. Estuaries, Tidal Marshes and Flats, and Deltas 



A. Definitions 



Estuaries and tidal marshes are river mouths and coastal areas where tides 

 and tidal currents play an essential part in sedimentation (Latin aestus means 

 tide), whereas in deltas the deposition results usually from a large excess of 

 load in the river stream at the place where the river meets the sea. True marshes 

 may exist, however, in some regions where the tidal range is low, for example at 

 Esbjerg, Denmark, or at Newport Bay, California, where it does not exceed 

 5 or 6 ft at spring tides (Jacobsen, 1956; Stevenson and Emery, 1958), or even 

 2 ft at Porto Vecchio in Corsica (Blanc and Picard, 1954), but the marshes tend 

 to be very poorly developed when the range falls below 3 ft. Marshes and tidal 

 flats are usually found on both sides of estuaries (Chesapeake Bay, Delaware 

 Bay, Gironde, Loire, Thames, Scheldt, Weser, Elbe, etc.), behind shingle- or 

 sand-spits (Norfolk in England, New England, Guianas, Guinea, etc.), or in 

 sheltered parts of bays (Bay of Fundy, Baie de I'Aiguillon and Marais Poitevin 

 in France). A special type consists of very wide tidal flats extending in front of 

 large reclaimed marshes: these flats are referred to as Watten (Dutch: Wadden) 

 in Dutch, German and Danish Friesland (Fig. 1); other watts are those lying 

 in the \A^ash in England, and in Mont Saint Michel Bay and Saint Brieuc Bay 

 in Brittany. On some watts, the distance between high and low spring water 

 marks may be as large as 10 nautical miles or even more (Johnson, 1925, pp. 

 504—588; Steers, 1946, chap, 14 and passim; Van Straaten, 1954; Gripp, 1956; 

 Russell, 1959, including examples from tropical and subtropical countries; 

 Phlipponneau, 1955; Verger, 1954). 



Deltas, which are generally piles of sediments at the mouths of rivers, are 

 more especially found in seas where the tidal range is insignificant, e.g. in the 

 Mediterranean (Rhone, Ebro, Po, Nice, etc.), in the Baltic (Weichsel), and in the 

 Gulf of Mexico (Mississippi), because these conditions are unfavourable to a 

 wide dispersal of sediment carried to the sea. They can, however, sometimes 

 coexist with large tidal ranges, for example in the Far East, provided that the 

 sedimentary discharge is very high, and true tidal marshes can occur in deltas 

 when the tidal range, or wind tides (surges), allow them to come into existence 

 (Mississippi). Most deltas have several distributaries, and the Lena River is 

 said to have no less than forty-five mouths; some, such as the Tiber, however, 

 have only one, but deserve their name because they protrude into the sea. The 

 classification of the mouth of the Senegal River is more controversial, since this 

 river has but one mouth and does not bulge the shoreline; the sediments have 

 only filled a former bay and continue to spread in the coastal region (see below). 

 Another transition t3rpe is the submarine delta which partially fills the outer 

 part of an estuary, as at the mouth of the River Loire (Guilcher, 1958, fig. 13, 

 p. 111). Submarine deltas are even found with no river at all: these are called 



[MS received April, 1960] 620 



