GULF OF MEXICO 



117 



1910 



1920 



1930 



1940 



1950 



Fi(;iRE 33. — Yearly sea level, Key West, Cedar Keys, 

 Pensacola, and Galveston. 



for the past 1 1 years, or at a rate somewhat larger 

 than at Key West. 



At Pensacola, the rise in sea level from 1924 to 

 1950 is very nearly half a foot. From 1924 to 

 1941 the rise was at the rate of about 0.01 foot 

 per year, but since then the rate appears to l)e a 

 little over 0.03 foot per year. 



At Galveston, the change in sea level from 1909 

 to 1950 is almost exactly 1 foot. From the dia- 

 gram of figure 33, sea level appears to liave risen 

 more or less uniformly from 1909 to 1937 about 

 0.4 foot or at the rate of 0.014 foot per year. 

 From 1938 to 1950 sea level rose 0.6 foot or at 

 the rate of 0.05 foot per year or at a rate more 

 than three times that of the earlier period. 



F'or the northern shores of the Gulf tlie obser- 

 vations availal)le incUcate a rise of sea level in re- 

 cent years but at dilfei'ent rates at the different 

 places. Furthermore, in tlie last (h-cade or two 

 the rise has been at a more rapid rale at all four 

 stations of figure 33. These matters clearly pose 

 questions of a geophysical nature which fall out- 

 side the restricted field of tides. 



It is of interest to note, however, that a change 

 of sea level at a given place may be due to one or 

 more of several different causes. Clearly, if a 

 coastal area is sidjsiding, sea level in that area will 

 rise relative to the coast, but a rise in sea level will 

 also occur along a stable coast if the volume of the 

 water in the open sea has increased. It appears 

 likely that such an increase, at a very slow rate to 

 be sure, is taking place at the present time 

 through an amelioration of the climate in high 

 latitudes, testified to by the recession of glaciers, 

 the melting waters of which are finding their 

 way to the open sea. Long-continued deposition 

 in the sea of river-borne material from the land 

 tends to decrease the depths of the sea and thus 

 cause a rise of sea level. Fiitally, even with 

 stability of the coast, of the volume of water in 

 the sea, and of the depths of the sea, a rise in sea 

 level at a particular place may conceivably occur 

 through a change in the seasonal distribution of 

 the direction and velocity of the winds. 



AVAILABILITY OF TIDAL DATA 



Tidal data for the Gulf of Mexico are available 

 in various forms. Tide Tables, East Coast, 

 North and South America, issued annually in 

 advance, . give daily predictions for a calendar 

 year of the heights and times of high and low 

 water for Key West, Tampa Bay, St. Mark's River 

 entrance, Pensacola, Mobile, Galveston, and Tam- 

 pico Harbor. In addition, these tables list Tidal 

 Differences and Constants for many other places 

 in the Gulf which permit ap{)roximate preilictions 

 to be made t>y reference to the above standard 

 stations. 



The Coast aiid Geodetic Survey also issues, in 

 looseleaf form, descriptions and elevations of the 

 tidal bench marks it has established in various 

 places along the United States coast of the Gulf. 

 These describe the location of each bench mark and 

 its elevation above mean low water. For each 



