*)0 TT. S. BTTREATT OF FISHERIES 



direction again. As a consequence of this movement of the flood tide, 

 we have the phenomenon of the tide rising at a rate of a little over 2 

 feet per hour at mid-flood tide with very low and irregular current 

 velocities. 



It was also noticed that, due to the configuration of Doboy Island 

 in jutting out into a narrow spit ot ballast rock on the southwest 

 corner, many eddies are set up along the western shore of the island. 

 One strong eddy was observed setting in a NNE. direction under the 

 wharf at Doboy at 5.15 p. m., April 24 (Table 4, fig. 18), while the 

 current in the main channel was setting in a general SSW. or S. by 

 W. direction. The eddy appeared to follow the shore line closely and 

 extended only 30 or 35 feet out into the channel. The place of obser- 

 vation appeared to be directly in the center of the eddy, and it is 

 highl}" probable that this accounts for the zero velocity of the surface 

 current at 5.30 p. m. and the variable velocities found from 3.30 to 7 

 p. m. Tests with the current meter showed that the eddy was a sur- 

 face movement that did not extend more than 2 feet below the sur- 

 face. 



CHANGES IN SALINITY DURING THE TIDAL CYCLES 



It can be seen from an examination of Figures 12 to 18 that salinity 

 curves folloAv in general the tidal curves, the minimum salinity oc- 

 curring about 1 hour after low Avater and the maximum salinity 

 shortly after high water. On August 30, 1928 (fig. 15), when the 

 river Avas at flood stage, the surface water was entirely fresh through- 

 out the tidal cycle. 



During the normal stage of the Altamaha River, the salinity at 

 neap tides is lower than it is during spring tides, when greater 

 volumes of ocean water enter the coastal region. This can be seen 

 by comparing Figures 12 and 16. On July 3, 1928 (spring tide), 

 the salinity at low slack water was 15.15 at the surface and 15.53 per 

 mille at the bottom; a week later (July 10, neap tide), it dropped 

 to 10.2G (surface) and 11.33 (bottom). Less significant changes were 

 observed at high Avater. On the same days the salinity at the top and 

 at bottom was 27.95 and 28.75, respectively (July 3, spring tide), 

 and 25.81 and 27.21 on July 10, neap tide. Flood conditions in 

 August and adverse weather conditions encountered in the spring 

 of 1929 prevented further study of changes in salinity during neap 

 and spring tides. 



AVhen observations were made in April, 1929, the waters around 

 Doboy Island were just clearing up from the flood of the Altamaha 

 River, which occurred during March, causing the displacement of the 

 salt water in the various rivers with fresh Avat«r. The Altamaha 

 River is formed by the junction of the Oconee and Ocmulgee Rivers, 

 both of which originate in the Piedmont Plateau. A river of this 

 class carries down a great amount of silt, which is precipitated upon 

 contact wuth salt water. The process of precipitation was going on 

 actively in April and May, and the demarcation between the clear, 

 greenish water of the sound and the red, silty water from the Darien 

 River, which is one of the mouths of the Altamaha River, was very 

 apparent. At Doboy Wharf the water Avas red and silty from an 

 hour after high Avater to half-flood tide. In Doboy Sound patches 

 of red water could be seen moving toward the sea on ebb tide. The 



