OYSTEB BOTTOMS IN MATAGORDA BAY. 51 



Reef the water ranges from 1 to 5 feet, being somewhat more shal- 

 low close to the roof than farther up the bay. Below Dog Island 

 the depth gradually increases to the maximum of If fool at the lower 

 limit of the survey. 



TIDES. 



At the time of the hydrographic survey of Matagorda Bay by 

 the Coast and Geodetic Survey no bench marks of a permanent na- 

 ture were erected, and for the present work it became necessary to 

 establish a new plane of reference. Matagorda was selected as the 

 most central and convenient locality, and a plain staff, reading from 

 to G feet, graduated in tenths, was fixed to a pile on the wharf 

 of Mr. Carr's oyster house at that place. The staff was protected on 

 three sides by boards 10 inches wide driven into the bottom at right 

 ano-les to one another and nailed. At the end of the season's work 

 a bench mark Avas established by driving a three-fourths-inch gal- 

 vanized iron pipe 10 feet long into the bottom close alongside the 

 tide gage, with its top coinciding with the reading of 1.5 feet on the 

 tide gage, the plane of reference adopted in this survey. This plane 

 of reference is the average of 31 low waters, from January 20 to 

 February 19, inclusive, and may be taken as the average low water 

 in the height of the oyster season. At this stage of the tide the crests 

 of Dog Island, Shell Island, and Mad Island reefs are exposed, and 

 there is less than a foot of water on the highest parts of Half Moon 

 Reef. For the purposes of this survey it was not deemed necessary 

 to establish secondary gages, for while it was recognized that the 

 barrier of Dog Island Reef would produce relative diversity in the 

 levels in the upper and loAver bay, the average error was compara- 

 tively slight and insufficient to have practical bearing upon the sub- 

 jects herein discussed. 



Owing to the remote and constricted connection with the gulf, 

 the tides in the part of the bay covered by this report are largely 

 independent of lunar influence, and it frequently happens that the 

 water level remains stationary throughout the day. The average 

 diurnal range during the period of tidal observations, from January 

 20 to May 11, inclusive, was less than 2J inches, and the maximum 

 change during twenty-four hours was 1.1 feet, from gage reading 

 2.2 feet at noon February 18, to gage reading 1.1 feet at 8 a. m. 

 February 19. 



The height of these tides is generally dependent upon the direc- 

 tion and velocity of the winds, southerly and westerly winds rolling 

 up the water above Half Moon Reef, and northerly and easterly winds 

 driving it out. In consequence of this, during the oyster season, 

 when there is a prevalence of wintry northers, the tides are in gen- 

 eral at their lowest, increasing in height as the spring advances and 

 southerly and southwesterly winds gain the ascendency. This is 



