Assessing the recovery of the Lower Hillsborough River

Hillsborough River in Temple terraceMarch 2020 – Over the last 20 years, the Southwest Florida Water Management District (SWFWMD) and the City of Tampa have been working closely on recovery of the Lower Hillsborough River. The recovery strategy, adopted in 2000 and revised in 2007, increased the minimum flow level (MFL) and required that for each five-year period through 2023. SWFWMD evaluates the strategy’s progress and implementation methods. As defined in the Florida Statutes, “the minimum flow for a given watercourse shall be the limit at which further withdrawals would be significantly harmful to the water resources or ecology of the area.” For the Lower Hillsborough River, that is currently set at 20 cubic feet per second (cfs) from July through March and 24 cfs from April through June.

Why is water flow so important? As an estuary – a lower salinity environment where the freshwater river meets the bay – the Lower Hillsborough River is relied upon by many species of fish in the early stages of their lives. This is critical for both recreationally and commercially sought species. Over the past few decades, the Lower Hillsborough River has faced growing water demands to serve Tampa’s increasing population. Gradually more and more water was withdrawn from the middle river above the City of Tampa Dam, causing freshwater flow to the Lower Hillsborough river to dwindle. Drained of its necessary freshwater to maintain estuary conditions, the Lower Hillsborough became a stagnant bay with high levels of salinity (the amount of salt dissolved in a body of water) and lower levels of dissolved oxygen. These conditions degraded the ecological health of the river and made it uninhabitable for estuary fish, plants, and aquatic animals that relied on it. The establishment of the minimum flow level is helping to maintain estuary conditions and breathe life back into the Lower Hillsborough River.

Danielle Rogers, Environmental Project Manager for SWFWMD, spoke at the River Board Technical Advisory Council (TAC) meeting on February 18, 2020, presenting the results of the draft 5-year Hydrobiological Assessment of the Lower Hillsborough River. The assessment evaluated water quality and quantity above and below the Hillsborough River Dam from October 1, 2012 through May 31, 2018. As shown in the report, the minimum flow level was met 88% of the time in 2,062 assessed days for the period October 2012 through May 2018 (some days were unable to be assessed due to equipment problems and maintenance). The four water sources used to achieve the minimum flow level are Sulphur Springs, Blue Sink, the Tampa Bypass Canal, and Morris Bridge Sink. Blue Sink was made operational March 2018 and is therefore only represented in the last two months of this report. And in 2019, installation of the sluice gate at the Tampa Bypass Canal came online. Although the 2019 data is not included in the time frame of this assessment and we only have two months of reporting for the Blue Sink, these additional tools are proving to be effective in the recovery strategy. In 2018 the flow target was achieved 93% of the time and in 2019 it was achieved 99% of the time, with only three days not meeting the flow target. With these new resources “it is expected that minimum flow targets, including the freshwater equivalent, are more likely to be achieved over the next five-year evaluation cycle.” (SWFWMD)

Looking at the biological conditions of the river, the system appears to be responding positively and aquatic communities are indicative of improved low-salinity conditions. SWFWMD found that the “benthic macroinvertebrate and fish communities seem to exhibit some evidence of a shift toward a community more reflective of purely freshwater habitats.” Furthermore, dissolved oxygen levels have increased “possibly contributing to increased species diversity and richness.” However, they acknowledge more biological sampling is needed to reach a definite conclusion.

SWFWMD is continuing to glean important insights from the data and has identified next steps as they begin work on the third and final 5-year evaluation cycle. Their revised action plan will introduce more biological sampling during spring and fall to better characterize the response of biological communities (planned for 2020 through 2023), assess the sustainable use of Sulphur Springs as a recovery-flow source (tentatively planned for 2022), monitor water quality and flow (ongoing), and work with City of Tampa on improved data-reporting methods to support timely status assessments.

The Technical Advisory Council is reviewing the five-year assessment to make a recommendation to the Hillsborough River Board in April 2020. Public comment on the draft assessment is encouraged and will be accepted up until April 15, 2020. SWFWMD will bring the evaluation before the District Governing Board in May 2020.

View more information, and read the full draft of the assessment.

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