Consensus Report

Review of the St. Johns River Water Supply Impact Study: Final Report (2011)

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The St. Johns River in Northeast Florida is under consideration as an additional water source for a large and rapidly growing segment of the state's population. In 2008, the river's Water Management District undertook a Water Supply Impact Study of the proposed water withdrawals and asked the National Research Council to review science aspects of the study as it progressed. This final report of the NRC Committee focuses on the seven ecological impact analyses conducted, it presents final thoughts about the hydrologic and hydrodynamic studies, and it provides some overall perspectives on the Impact Study.

The report finds the District performed a competent job in relating predicted environmental responses, including their magnitude and general degree of uncertainty, to the proposed range of water withdrawals. Two of the District’s workgroups achieved state-of-the-art science, according to the Committee, with potential uses beyond this study; those workgroups focused on hydrology and hydrodynamics and on submersed aquatic vegetation.

Key Messages

  • The overall strategy of the Water Supply Impact Study and the way it was implemented were appropriate and adequate to address to goals the District established for the study.
  • The District scientists welcomed the Committee’s recommendations and implemented them when feasible, thus overcoming many of the limitations noted by the Committee early in the project. The Committee commends the District workgroups for their careful and thoughtful responses to its suggestions.
  • The District’s final report should acknowledge such critical issues as include future sea-level rises, population growth, and urban development. Their predicted effects on water levels and flows in the river are greater in magnitude than the effects of the proposed surface water withdrawals, but they have high uncertainties. Using an adaptive management strategy, the District should plan to run its models with more recent rainfall and land use records.
  • The Committee continues to be somewhat concerned with the conclusion that the water withdrawals considered in the WSIS will have few deleterious ecological effects. This conclusion was based on the model findings that increased flows from upper basin projects and from changes in land use (increases in impervious areas) largely compensated for the impacts of water withdrawals on water flows and levels. Although the upper basin projects are positive insofar as they will return land to the basin (and water to the river), the same cannot be said about increased urban runoff, the poor quality of which is well-known.