The Determination of Minimum Flows for Sulphur Springs, Tampa, Florida September 28, 2004 DRAFT The Determination of Minimum Flows for Sulphur Springs, Tampa, Florida prepared by: The Resource Conservation and Development Department of the Southwest Florida Water Management District 2379 Broad Street Brooksville, FL. 34604-6899 DRAFT September 28, 2004 DRAFT TABLE OF CONTENTS List of Tables ………………………………………………………………………………iv List of Figures ……………………………………………………………………………...v Acronyms and Definitions………………………………………………………………..xi Executive Summary ……………………………………………………………………..xiii Acknowledgments………………………………………………………………………...xvi Chapter 1 - Purpose and Background of Minimum Flows and Levels 1.1 Overview ........................................................................................... 1-1 1.2 Legislative Directives......................................................................... 1-1 1.3 Application to MFLS for Sulphur Springs........................................... 1-3 1.4 Content of Remaining Chapters ........................................................ 1-5 Chapter 2 - Physical and Hydrologic Characteristics of Sulphur Springs 2.1 Introduction........................................................................................ 2-1 2.2 Current physical setting of the spring pool and run............................ 2-3 2.3 Water Supply Use of Sulphur Springs ............................................... 2-8 2.3.1 Recent improvements to the diversion facilities at Sulphur Springs ............................................................................................ 2-11 2.4 Hydrogeologic and Flow characteristics of Sulphur Springs............ 2-14 2.4.1 Geologic Setting .............................................................................. 2-14 2.4.2 Springflow characteristics................................................................ 2-15 2.5 Water Quality of Sulphur Springs .................................................... 2-19 2.5.1 Data sources ................................................................................... 2-19 2.5.2 Trends in specific conductance ....................................................... 2-21 2.5.2.1 Effects of gate operations and withdrawals on specific conductance .................................................................................... 2-24 2.5.2.2 Effects of recent smaller withdrawal rates ....................................... 2-30 2.5.3 Concentrations and trends of other constituents ............................. 2-32 Chapter 3 – Ecological Resources of Sulphur Springs and the Lower Hillsborough River 3.1 Introduction........................................................................................ 3-1 3.2 Salinity and water level records in the spring and river system ......... 3-1 3.3 Tidal water level fluctuations in the spring run................................... 3-2 3.4 Salinity in the spring run and response to withdrawals from the spring pool......................................................................................... 3-5 i DRAFT 3.5 Dissolved oxygen concentrations in the spring run and response to withdrawals from the spring pool. ...................................................... 3-9 3.6 Other water quality characteristics of the spring run...................... ..3-10 3.7 Biological habitats and communities in the spring run..................... 3-11 3.7.1 Benthic habitats............................................................................... 3-11 3.7.2 Shoreline vegetation........................................................................ 3-11 3.7.3 Benthic macroinvertebrates............................................................. 3-12 3.7.3.1 1997 Macroinvertebrate survey ....................................................... 3-12 3.7.3.2 Invertebrate collections from 2000 to 2003...................................... 3-13 3.7.3.3 Results of FWC qualitative samples................................................ 3-14 3.7.3.4 Percent composition in various habitats ......................................... 3-17 3.7.3.5 Quantitative samples from May 2000, November 2001, and December 2003........................................................................ 3-18 3.7.3.6 Possible factors affecting changes in the macroinvertebrate community ....................................................................................... 3-21 3.8 Fish populations in the spring run.................................................... 3-22 3.9 Relationships of Sulphur Springs discharge to the hydrology, water quality and ecology of the Lower Hillsborough River....................... 3-26 3.9.1 Hydrology of the Lower Hillsborough River ..................................... 3-26 3.9.2 The effects of flows from Sulphur Springs on salinity distributions in the Lower Hillsborough River ................................................................ 3-31 3.9.2.1 Empirical salinity data plots ............................................................. 3-33 3.9.2.2 Hydrodynamic salt transport model of the river ............................... 3-36 3.9.3 Relationships of freshwater inflows on dissolved oxygen concentrations in the lower Hillsborough River................................ 3-38 3.9.4 Effects of low flows on salinity distributions and dissolved oxygen concentrations between the dam and Sligh Avenue........................ 3-42 3.10 Biological characteristics of the Lower Hillsborough River and relation to the management of Sulphur Springs ........................................... 3-48 3.10.1 Intertidal vegetation ......................................................................... 3-48 3.10.2 Benthic macroinvertebrates............................................................. 3-49 3.10.3 Fishes.............................................................................................. 3-50 3.11 Thermal characteristics of the spring run and the lower river and use by cold sensitive species ................................................................. 3-52 3.11.1 Thermal characteristics of Sulphur Springs pool and run ................ 3-53 3.11.2 Thermal characteristics of the Lower Hillsborough River................. 3-56 3.11.3 Vertical and diurnal variations in river water temperatures .............. 3-60 3.11.4 Temperature variability and relation to biological use...................... 3-60 Chapter 4 – Technical Approach for Determining Minimum Flows for Sulphur Springs 4.1 Overview ........................................................................................... 4-1 4.2 Consideration of structural alterations to the determination of minimum flows .................................................................................................. 4-1 4.3 Consideration of seasonal variations of springflow and relationships to flows from the Hillsborough River reservoir ....................................... 4-2 4.4 Selection of management goals and ecological indicators for evaluating the effects of reduced flows ........................................ 4-3 ii DRAFT 4.5 Goal 1 – Minimize the incursion of high salinity river water into the upper spring run. ......................................................................... 4-4 4.6 Goal 2 - Maintain low salinity habitats in the Lower Hillsborough River .................................................................................................. 4-5 4.6.1 Distribution and volume of <4 ppt and <11 ppt salinity zones as ecological indicators in the lower river ............................................... 4-6 4.7 Goal 3 – Maintain a thermal refuge for Manatees in the winter ......... 4-8 Chapter 5 - Results of the Minimum Flows Analysis 5.1 Introduction........................................................................................ 5-1 5.2 Criterion 1 – Maintain low salinity habitats in the upper spring run.... 5-1 5.2.1 Criterion 1 data sources .................................................................... 5-1 5.2.2 Controlled release experiments......................................................... 5-1 5.2.3 Salinity response during the controlled release experiments............. 5-3 5.2.4 Salinity response at the continuous recorder in the upper spring run to variable flows........................................................................... 5-8 5.2.4.1 Tide stage effects on salinity incursions .......................................... 5-16 5.2.5 Relations of springflow to water levels in the upper spring run........ 5-18 5.2.6 Salinity response in the lower spring run ......................................... 5-21 5.2.7 Other potential effects of reduced flows. ......................................... 5-22 5.3 Criterion 2 – Maintain low salinity habitats in the Lower Hillsborough River ................................................................................................ 5-23 5.3.1 Salinity distributions for different minimum flow scenarios predicted by a hydrodynamic salt transport model .......................... 5-23 5.4 Summary of incorporating criteria 1 and 2 low-salinity factors into a minimum flow for Sulphur Springs................................................... 5-32 5.4.1 Application of minimum flows based on criteria 1 and 2 .................. 5-37 5.5 Criterion 3 - Maintain a thermal refuge for Manatees ..................... 5-38 5.5.1 Purpose ........................................................................................... 5-38