60 OYSTER BOTTOMS IN MATAGORDA BAY. 



The tendejncy of the strongest upward currents carrying the water 

 from the gulf to hug the peninsula shore also operates to produce a 

 higher density in that part of the bay. As the densities taken by the 

 Fish Hawk at this time were about 50° below the normal estab- 

 lished by the series, it is probable that the averages for the months of 

 January, February, March, and April in this region were from 30° 

 to 40° higher than above indicated. Below Mad Island "Reef the 

 fresh water discharged through Dog Island channel having been de- 

 flected southward by two long projecting barriers of oysters extend- 

 ing from the northwest and commingled by currents and wave action 

 with the denser waters from the lower part of the bay, the disparity 

 in density between the two shores is much less marked, the respective 

 averages of the three zones, beginning at the northwest shore, being 

 1. 01-10, 1.0168, and 1.0163. These readings were obtained between 

 April 22 and 28, and as the Fish Hawk observations were then about 

 10° above the established normal, the local readings should be re- 

 duced by that amount in order to obtain the probable average between 

 January 1 and May 1, 1905. 



The " normal " referred to in several places above is the average of 

 381 density observations made at the Fish Hatch anchorages from 

 January 1, 1905, to May 7, 1905, inclusive. The monthly averages 

 are as follows: January, 1.0124; February, 1.0154; March, 1.0134; 

 April, 1.0092, and May, 1.0100. The average daily observations are 

 shown graphically in the upper curve on plate x, an inspection of 

 which will show that the densities were sometimes fairly uniform 

 for several days in succession, but frequently exhibited sudden and 

 violent fluctuations. A study of these fluctuations shows that thev 

 are in large measure conditioned by the tides, and the latter are in 

 turn, as has been previously stated, mainly influenced by the winds. 

 A northeast wind, therefore, lowers the tide and decreases the density, 

 while a southwest wind has the opposite influence. To illustrate this 

 influence of the tides upon the density a tidal curve has been prepared 

 showing the mean daily height of water at Matagorda above or below 

 the plane of reference. It will be seen at once that there is a general 

 coincidence of the two curves ; whenever the tidal curve rises or falls 

 abruptly there is a more or less synchronous rise or fall in the densi- 

 ties. The explanation is that whenever there is a low tide after a 

 period of tidal elevation the current sets down the bay, carrying the 

 fresh water discharged by the streams into the region below Dog- 

 Island Reef, whereas a high tide after a period of tidal depression 

 backs the salt water from the gulf toward the head of the bay. Of 

 course, these phenomena are related solely to what has taken place 

 immediately prior to the time of observation and have no bearing 

 upon more remote facts. For instance, the tides of the middle of 

 April were higher than any of those of January and February, yet 



