Review of the St. Johns River Water Supply Impact Study Final Report (2012) / Chapter Skim
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of the National Academies was asked to review the progress of the WSIS, including such scientific aspects as hydrologic and water quality modeling and how river withdrawals will affect wetlands, biogeochemical processes, plankton, benthos, the littoral zone, fish, and wetlands wildlife in the basin. For two and half years, the WSTB Committee has followed the activities of eight District workgroups as they modeled the relevant river basins, determined the criteria to evaluate the environmental impacts of water withdrawals, evaluated the extent of those impacts, and coordinated with other ongoing projects.
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continue developing the Hydroperiod Tool and analyzing water level data from transects used to develop regulations on minimum flows and levels (MFLs) to determine the correspondence between river stage and wetland hydroperiod and thus the potential responses of different wetland types to water withdrawals.
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. Wetlands The wetlands workgroup was tasked with assessing the potential effects of surface water withdrawals on floodplain wetlands, specifically changes to vegetation communities that might result from altered hydrology and/or changing salinity regimes.
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Using data on release rates gathered from the literature and very limited data from their own field and laboratory studies, the workgroup assessed the changes in DOC, TP, NH4+-N, and TKN loading, as well as changes in dissolved oxygen concentrations, that would accompany water withdrawals and associated lowering of water levels. Only segment 8 received the full analysis, and the workgroup found that the impacts of an extreme withdrawal scenario would be negligible.
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Johns River unless strong management efforts are undertaken to control nutrient export. This secondary effect of the proposed water withdrawals could be as much a problem as salinity and water levels in determining the fate of SAV in the St.
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The workgroup combined this information with H&H model results on water levels to make predictions, using professional judgment and a "weight of evidence" approach, about the potential effect of water withdrawals on benthos. Their final predictions for the extreme withdrawal scenarios ranged from negligible to moderate.
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under extreme withdrawal scenarios. Overall, the freshwater fish workgroup posed appropriate questions related to potential impacts on fish assemblages and addressed them as much as available data would allow.
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The estuarine fish workgroup is commended for modifying, in response to the Committee's input over the course of two years, their approach to the complex issue of how fish will respond to changes in flow and salinity. As with the freshwater fish, the estuarine fish workgroup posed appropriate questions related to potential impacts on fish assemblages and addressed them as much as available data would allow.
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Under the extreme withdrawal scenario, major impacts were predicted for estuarine wildlife in segment 1, moderate impacts were predicted for estuarine wildlife in segment 2, and moderate impacts were predicted to freshwater wildlife in segments 2, 7, and 8. The wetlands wildlife analysis was limited by the lack of quantitative, speciesspecific information on the response of wildlife to altered hydrology and salinity.
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These issues include future sealevel rises and increased stormwater runoff and water quality degradation of surface runoff engendered by future population growth and increases in impervious area and pollutant generation associated with urban development. The predicted effects of sea level rise and land use change 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.
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