54 OYSTER BOTTOMS IN MATAGORDA BAY. 



the tidal irregularities noted in the preceding chapter, and any obser- 

 vation made would have been applicable to the time of observation 

 solely. It was considered, therefore, in view of the multiplicity of 

 factors requiring investigation, that the time necessary for this work 

 could be more profitably devoted to other fields. 



Though lunar influence is felt to a slight extent through the con- 

 nection of the bay with the gulf at Pass Cavallo, for all practical 

 purposes the tides, and therefore the currents, are under the domi- 

 nation of meteorological conditions. Northeast w T inds set up currents 

 running down the bay and southwest winds produce the opposite 

 effect, and the velocity depends largely upon the velocity of the wind. 

 On calm days the set and strength of the currents is conditioned by 

 the direction and velocity of the wind on preceding days; in other 

 words, upon the character of the movement requisite to establish an 

 equilibrium of level between the different parts of the bay. During 

 calms after northeasters the set of current is toward the head of the 

 bay, after southwesters toward the mouth. Another meteorological 

 factor which affects the water level, and therefore the currents, is the 

 amount of rainfall and the discharge into the upper bay through the 

 Colorado River, Caney Creek, and smaller streams. When the dis- 

 charge is heavy there is a general set toward the mouth of the bay, 

 and this may continue even while the tide is rising, a common feature 

 of tidal phenomena in estuaries. Owing to the small average diurnal 

 change of water level, as shown by the daily observations, the currents 

 in the bay are necessarily weak, excepting in the channels through 

 Dog Island Reef, where there is usually a strong flow, often in one 

 direction for several days in succession. On most of the oyster beds 

 of the Atlantic coast the tidal change is between 2 and 6 feet twice 

 daily, and it will be at once seen that the currents must be of much 

 greater velocity than in Matagorda Bay,- where there is an average 

 diurnal change of less than 2-J inches in the entire twenty-four hours 

 and frequently no change at all. In the Chesapeake Bay oyster 

 region, where the tide ranges from 1J to 2^ feet in average height, 

 there is a maximum current of from 0.4 to 1.5 knots, according to 

 locality, four times each day, and the slack water at any time is of 

 short duration. At Cherrystone Light the average daily current is 

 0.9 knot, and it is at almost that velocity within an hour of slack 

 water, either flood or ebb. 



The importance of these currents to the oyster industry is consid- 

 erable. They scour and keep clean the shells or other material offer- 

 ing surfaces for the attachment of spat; they distribute widely the 

 minute swimming embryos or fry and make possible a set of spat 

 in places distant from the location of the parent oysters, and, finally, 

 they bring constantly renewed supplies of food-laden water within 

 the influence of the weak currents which the oyster itself produces 



