24 Routes from the Sea 



conflict between tide and river current. When the tide flows, salt 

 water may invade the mouth of a river, especially when a favorable 

 wind drives the surface water upstream, but under ordinary con- 

 ditions the surface of a river is fresh, even for some distance out 

 into the ocean beyond the mouth. The yellow waters of certain of 

 the great rivers which flow eastward along the coast of China give 

 approaching vessels notice of their presence before land can be 

 seen. The estuary of a river which flows into a sheltered bay or 

 sound usually shows a graded series of salinities, often with exten- 

 sive areas of brackish water which may remain relatively constant. 

 Bourn (Weese, 1928) on the Atlantic coast of the United States 

 studied salinities at 26 stations in a brackish-water area. He says: 

 "There is no correlation between salinity and the rainfall. . . . Since 

 there are no meteorological tides in these inland waters, the salt 

 content of Back Bay and Currituck Sound is governed by the 

 force and direction of the winds. The influx of salt water in 

 varying amounts has disturbed natural conditions and consequently, 

 has greatly altered the aquatic life of the region." 



In some respects, marine animals which burrow have a better 

 chance to become adjusted to life in estuaries than those which live 

 in the water above. "The water retained in the muddy foreshore 

 of an estuary at low tide was more saline than the estuary water 

 itself at the same distance from the sea. The retention of salt by 

 the bottom and shore deposits may be a factor favoring the growth 

 of burrowing animals in the central part of an estuary" (Alexander, 



Fig. 4. Mangroves (Avacinea) in a Colombian estu- 

 an/. The mud in which they grow contains little or 

 no oxygen and many breathing roots grow up into 

 the air. The small tree in the left foreground shows 

 prop roots. 



