Chapter XIII 



rides in Estuaries 



1. Observations 



The tides of the open ocean cause oscillations in estuaries which make the 

 water level rise and fall with the tides. The place up to where the effect of 

 the tides is noticeable is called the tide mark. 



That part of the river which is subject to the tides of the oceans and which 

 stretches from the estuary to the tide mark is here called the river tide zone 

 or briefly tide zone. The Amazon River seems to have the longest tide zone. 

 According to La Condamine, the tidal mark lies near Obidos, i.e. 850 km 

 from the opening; however, Bates (1863, p. 133; see also Schichtel, 1893, 

 p. ICO) still found tidal effects in the Cupari, a minor tributary of the Tapajoz 

 ending near Santarem, which is at a distance of 870 km from the ocean. 

 In the case of the Congo River the tide mark is only 170 km from the Atlantic 

 Ocean, because the rapids near the estuary prevent a further extension of the 

 tide zone. In the Yangtse Wuhu River the length of the tidal zone is 500 km; 

 in summer, the tidal effects are completely suppressed by the high current 

 velocities in the swollen river (Hofmeier, 1901). 



The tidal wave penetrating into the river is subjected to some characteristic 

 modifications especially by the decrease in depth and the ensuing increased 

 friction and by the river flow itself. These influences must be considered chiefly 

 as being of a hydraulical nature and are much more subjected to the external 

 conditions than the purely hydrodynamical occurrences. 



Systematic investigations of the tide zone were made for a few European 

 rivers only, and not all observational data have the reliability one would 

 desire. The most important feature is the shape of the tidal curve. Whereas 

 it is still entirely symmetrical in the open ocean in front of the estuary, the 

 curve becomes asymmetrical in the river. The water-level shows a quicker 

 rise and a slower fall, so that the duration of the flood is shorter than that 

 of the ebb. Table 73 gives a series of data for the Elbe and the Weser, ac- 

 cording to Franzius (1880, p. 299); for the Gironde and Garonne according 

 to Comoy (1881); for the St. Lawrence River according to Bell Dawson 

 (1824, p. 141). Figure 193 gives as examples tidal curves of the Elbe for three 

 periods of 12 lunar hours, whereas Fig. 194 shows tidal curves for Father 

 Point and Quebec at the St. Lawrence River for the period from 6 to 9 Oc- 

 tober, 1896, which illustrated clearly the asymmetry. 



