Currents in a Strait 



539 



investigation of Swedish rivers found that the outflow of river water in the estuary 

 was accompanied by an inflow of sea water in the lower layers. Thus, at the mouth of 

 the Gotaelf into the Elfsborgsfjord, there was a strong compensation requirement for 

 the outflowing surface water which could not be satisfied by inflow from the sides. It 

 therefore gave rise to upweUing motions from below. The consequent reverse deep 

 current was clearly shown by the salinity distribution at different depths and could also 

 be shown experimentally by drift buoys. The rising water was both more saline and 

 more transparent than the sewage-laden river water. Figure 250 shows the salinity distri- 

 bution along a longitudinal section; the upstream directed lower current is demon- 

 strated clearly by the 20%o isohahne. 





Fig. 250. Vertical distribution of salinity in the river mouth of the Gotaelf. (I) 5 August 



1875; (II) 19 February 1890. 



A theoretical investigation of the occurrence of lower currents of this type in river 

 mouths (estuaries) was made by Ekman (1899) using principles similar to those used 

 in the theory of currents in sea straits. He found that under normal conditions there 

 were no currents carrying sea water upstream, but that such a current was formed 

 immediately if there was a tangential force acting on the sea surface. The shallower the 

 water, the greater must be the tangential pressure in comparison with the surface 

 (river) velocity in order to allow for the generation of a compensation current in the 

 deep water. River water entering an estuary flows on top of the sea water partly 

 because of its inertial momentum and partly because of its lower density. It thus 

 exerts the tangential pressure on the lower layer which favours the compensation 

 current. 



The momentum and the density are apparently, however, of less importance than 

 the density difference between the upper and lower layers and turbulent mixing of the 

 two water types. 



