HYDROLOGICAL CHARACTERISTICS OF TAMPA BAY TRIBUTARIES 



165 



GENERAL DESCRIPTION OF THE AREA 



The Hillsborough, Alafia, Little Manatee, and 

 Manatee Rivers are relatively small and short, 

 ■with sources less than 70 miles inland (fig. 1). 

 These rivers are the main arteries of the Tampa 

 Bay drainage basin, and all four river mouths are 

 subject to tidal action. The mean range of tide 

 is 1.9 feet at the lower end of Tampa Bay, 2 feet 

 at Port Tampa, and 2:2 feet at Tampa. Tiie ex- 

 treme ranges at Port Tampa and at Tampa are 

 2.5 and 2.9 feet, respectively. Strong nortlierly 

 winds lower the water level about 2 feet while 

 strong southwesterly winds raise the water level 

 about 1.5 feet. 



Five gaging stations (fig. 1) are maintained in 

 the rivers by the LT.S. Geological Survey, Branch 

 of Surface AVaters, for the purpose of obtaining 

 river discharge data. 



Outstanding geological characteristics of the 

 area are large phosphorus and limestone deposits. 

 The major deposits containing phosphate are 

 the Hawthorn (Miocene), Alachua (Pliocene), 

 and Bone Valley (Pliocene) fonnations (fig. 2). 

 Although the soils are high in lime and phos- 

 phorus, they are poor in copper, zinc, and manga- 

 nese (Fuestel and Byei-s, 1933). The copper level 

 has been supplemented in recent years in the citrus 

 and truck farming areas by the addition of ferti- 

 lizers of high copper content. Patches of peat and 

 muck soils are scattered throughout the Tampa 

 Bay drainage basin, altliough not to the extent ob- 

 served in the Everglades. The detailed geologj' of 

 the investigation area has been discussed by Cooke 

 (1945). 



The rainy season in the Tampa Bay area usually 

 extends from June to October, and the streams 

 reach seasonal high stages during late summer and 

 early fall. Tlie mean annual precipitation com- 

 piled from five centei-s (Bartow, Bradenton, Lake- 

 land, Plant City, and Tampa) for the past 29 years 

 (1931-59) is 53.5 inches. 



The prevailing winds are easterly. The maxi- 

 mum wind velocity in the past 57 years was 75 

 miles per hour, while the mean velocity was 8.4 

 miles per hour. The Tampa Bay area lias sub- 

 tropical temperatures, generally witli maxima in 

 August and minima in January. 



RESULTS 



The water temperature ranged from 15.0° to 

 30.5° C. during this investigation. The seasonal 

 decline in temperature in all rivers began in No- 

 vember during both observation years, reaching 

 the seasonal minimum in December, which was 

 followed by a sharp rise in January (figs. 3, 4, 5, 

 and 6). From January tlirough March, the tem- 

 perature increased slightly in the Little Manatee 

 River. In the Hillsborough, Manatee, and Alafia 

 Rivers, a slight increase was recorded from Janu- 

 ary through February, followed by a drop in 

 March. From March to May, temperature in- 

 creased sharply in all rivers and remained 

 variably high from May to October. 



The frequency distribution of the temperature 

 diti'erences between surface and bottom ranged 

 from 0.0° C. to 0.5° C. in 83.3 percent of the sam- 

 ples; 0.6° C. to 1.0° C. in 10.9 percent of the sam- 

 ples; and 1.1° C. to 3.1° C. in 5.8 percent of the 

 samples. This distribution indicates close simi- 

 larity between the surface and bottom tempera- 

 tures. 



The maximum salinity, 24.83 Voo, was observed 

 at the bottom of station 36 while a minimum of 

 0.04 Voo was recorded at stations 34, 35, 30, 31, and 

 32 (figs. 3, 4, 5, and 6). The salinity values in- 

 dicated that station 30, Hillsborough River, and 

 station 34, Little Manatee River, possessed lim- 

 netic characteristics throughout the obsei'vation 

 period. Tlieir respective ranges v.ere 0.04 *'/oo to 

 0.32 Voo and 0.04 Voo to 0.48 Vo.,. At all remaining 

 stations much greater variation in salinity was 

 noted. 



Total annual precipitation in the Tampa Bay 

 area was 57.4 inches in 1958 and 82.1 inches in 

 1959. The precipitation regime for the Hills- 

 borough and Alafia drainage areas ditl'ei-ed from 

 tliat of the Little ]\Ianatee and Manatee River 

 areas, the latter having more rain during the sum- 

 mer months (figs. 3, 4, 5, and 6). The early onset 

 of the rainy season in Marcli was most pronounced 

 in the Hillsborough and Alafia areas. 



In the Hillsborough and Alafia River drainage 

 areas, maximal discharges began in March and, 

 with the exception of a decline in May, remained 

 variably liigh tlirougli October for the Hills- 

 borough, and through September for the Alafia 



