SWFWMD releases Hydrobiological Assessment

The Hillsborough River Dam, showing the middle and lower Hillsborough River.

The Hillsborough River Dam, showing the middle and lower Hillsborough River.

Mr. Douglas A. Leeper, Chief Advisory Environmental Scientist with the Water Resources Bureau of the Southwest Florida Water Management District (SWFWMD), gave a presentation to the River Board’s Technical Advisory Council on June 16 on SWFWMD’s Hydrobiological Assessment of the Minimum Flows for the Lower Hillsborough River. The recovery strategy that was adopted with the minimum flow rule for the Lower Hillsborough River in 2007 required that in 2013, and for each five-year period through 2023, SWFWMD evaluate the health of the river achieved from implementation of the recovery strategy for the previous five years. This first five-year report addresses the objective by examining changes in the hydrobiological characteristics of the lower river in response to the minimum flows that were implemented between 2002 and 2013.

Mr. Leeper explained the following from the report: “The implementation of minimum flows for the Lower Hillsborough River, below the Hillsborough River Dam, began in the spring of 2002. Until 2008, minimum flows were comprised solely of diversions from Sulphur Springs, usually at a rate of 10 cubic feet per second (cfs). Beginning in 2008, diversions from Sulphur Springs were accompanied by freshwater releases from the Hillsborough River Reservoir / Tampa Bypass Canal (TBC) system. Beginning in the spring of 2012, total minimum flow rates from 18 to 27 cfs were achieved because modifications to the pumping facilities at Sulphur Springs allowed for the diversion of greater quantities of springflow to the base of the dam. The minimum flow rates achieved after the spring of 2012 met the minimum flows for the lower river on many days.”*

“A principal goal of the minimum flow rule for the Lower Hillsborough River is to extend a zone of oligohaline water (i.e., water with salinity < 5 psu) from the base of the Hillsborough River dam toward Sulphur Springs. The results of this study demonstrate the benefit of increased minimum flows for achieving that goal. Based on data from continuous recorders upstream of Sulphur Springs, it appears that minimum flow rates in the range of 23 to 26 cfs produce significantly lower and less variable salinity conditions compared to a minimum flow rate of 20 cfs.” *

The report concludes: “In general, the findings of the study supported the validity of the minimum flows that were adopted for the Lower Hillsborough River in 2007.”*

*Source: A Hydrobiological Assessment of the Phased Implementation of Minimum Flows for the Lower Hillsborough River, Southwest Florida Water Management District and Atkins North America, Inc., March 3, 2015.

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