Withdrawals and operating schedules for the three instream 
reservoirs have resulted in the significant reduction of dry season flows 
in those rivers and, periodically, the virtual elimination of flows past 
the dams entirely. This is illustrated in Figure 8, where monthly 
withdrawals and discharge from the Hillsborough River reservoir are 
plotted for October 1982 to September 1986. During this period discharge 
from the reservoir averaged 325 cfs, but there were 547 days when daily 
discharges were less than 20 cfs. Outflows from the Manatee and Braden 
Rivers are similarly affected by extended low or zero flow periods. 
Hillsborough River 
Monthly Discharge and Withdrawals from Reservoir (Oct. 1982-Sept. 1986) 
Figure 8. Monthly discharge and withdrawals from the Hillsborough River 
reservoir. 
The impoundment and utilization of these rivers flowing to Tampa 
Bay has certainly impacted the downstream estuarine environments. The 
most conspicuous effect of instream reservoirs is the elimination of 
movement past that point by migratory organisms such as fishes. This is 
particularly detrimental in estuarine areas where the juveniles of many 
marine species migrate upstream to utilize low and moderate salinity 
habitats. The Braden River dam was built in the estuarine zone of that 
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