1985 conditions, as well as the percentage change. In both 1880 and 
1985, computed circulation in zone 3 is less than in either zone 2 or 
zone 4, although not as pronounced as 1985. It is not known whether the 
circulation minimum in zone 3 influences the overall rate of constituent 
flushing from Tampa Bay to the Gulf of Mexico. Circulation differences 
range from a decrease of about 10 percent in zone 1 at the mouth of the 
bay to an increase of 275 percent in Hillsborough Bay (zone 6). The 
large computed increase in Hillsborough Bay circulation was investigated 
in more detail by Goodwin (in press) using a 500-ft grid model having 
nine times more spatial resolution than the Tampa Bay model. This model 
assumes no difference between 1880 and 1985 freshwater inflow. It does, 
however, include a total bay volume increase of about 10 percent and a 
tidal prism decrease of 6 percent during the same period. 
Table 2. Circulation changes in Tampa Bay between 1880 and 1985 by zone. 
Circulation 
Zone 
in cubic 
1880 
feet per second 
1985 
Percent 
Change 
1 
45,500 
41,100 
- 9.7 
2 
10,400 
13,400 
+28.8 
3 
4,900 
6,300 
+28.6 
4 
8,600 
7,800 
- 9.3 
5 
2,700 
3,700 
+37.0 
6 
400 
1,500 
+275.0 
The smaller grid model confirmed that dredge and fill construction 
of channels, islands, and shoreline fills between 1880 (Figure 7a) and 
1985 (Figure 7b) caused a dramatic increase in the number and intensity 
of circulation features in Hillsborough Bay (Figure 8). A comparison 
plot of circulation versus distance from the mouth of Hillsborough Bay 
(Figure 9) also demonstrates large circulation increases in most parts of 
the bay between 1880 and 1985. 
In addition to obvious circulation dissimilarities, the 500-foot 
grid model also revealed what is believed to be an important similarity 
in the Hillsborough Bay circulation patterns of 1880 and 1985. There is 
a tendency for tidally averaged water motion to flow in a seaward 
direction along the shallow bay margins and in a landward direction in 
the deeper central part of the bay. Although unconfirmed by direct flow 
measurements, this computed pattern is at least partially substantiated 
58 
