Freshwater flows are the lifeblood of the lower Hillsborough River. This area is an estuary, where saltwater and freshwater meet. As such, it is an important nursery ground for numerous commercially and recreationally important fish, crabs, and other species. With the growing population of Tampa taking more and more drinking water from the river, there has been less and less water over time going down to the lower river. That is why there is a regulatory minimum flow requirement for the lower Hillsborough River. Flows are provided from the middle river through the dam during seasonally high rainfall and high flow. During dryer periods where there may be little or no flow at the dam, the City of Tampa manages this minimum flow by moving water from lower on the river at Sulphur Springs to the base of the City of Tampa Dam. Water can also be pumped from the nearby “Blue Sink” to the base of the dam to provide freshwater flow.
Although we saw a relatively wet winter, spring is starting very dry with little rainfall. The Tampa Bay area is currently experiencing severe drought conditions, and these conditions are expected to persist for some time.
Also see: https://www.weather.gov/tbw/droughtinfo
The United States Geological Survey (USGS) monitors water levels in the City of Tampa Reservoir on the Hillsborough River and water discharge past the dam.
Water levels are dropping in the reservoir and are significantly below the top of the dam. USGS data also currently shows no water discharge past the dam. With continued withdrawals from the reservoir for potable use, it is under these conditions that supplemental water is used to maintain freshwater flows on the lower Hillsborough River.
The current rules require that between 20 and 24 cubic feet per second (cfs) of freshwater flow must be maintained below the City of Tampa’s Dam, depending upon the season. There is a provision in the rule that if the water used to maintain this flow contains salt and is not “fresh water,” the amount of flow must be increased to account for a “freshwater equivalent.” It has been known for some time that the water used from Sulphur Springs to meet the freshwater flow is becoming saltier over time, largely because of water withdrawals from the spring. The last time this issue was studied, SWFWMD established that the appropriate freshwater flow equivalent was 23 and 27 cfs, seasonally.
It is through this monitoring and supplemental water system that the Hillsborough River can continue to be used for water supply while protecting the important estuary that is the lower river.