Minimum Flows and Levels

St. Johns River Water Management District: Minimum Flows and Levels – Water Resource and Human-use Values Assessment for Lake Poinsett

HSW Engineering, Inc., was contracted by St. Johns River Water Management District (SJRWMD) to evaluate whether the hydrologic regime defined by the proposed State of Florida Minimum Flows and Levels (MFLs) will protect the ten natural resources and environmental values for the St. Johns River at Lake Poinsett.  The District proposed three annual minimum surface water flows and levels (MFLs) at SR-520: minimum frequent high, minimum average, and minimum frequent low.  The proposed flows and associated stages are based on detailed conditions observed at multiple transects along this section of river.  Potential water supply yields evaluated by others determined that a two-tiered withdrawal scenario would provide the maximum available water and would be in compliance with the proposed MFLs at SR-520.

HSW’s approach was to evaluate each WRV (Water Resource Value) for specific indicators of protection and then to use stage-frequency characteristics to determine if the ten WRVs would be protected under the proposed MFLs regime.  More specifically, the return interval (i.e. the inverse of the frequency of occurrence) of each specific indicator was evaluated under baseline conditions (i.e., historic flow conditions) and under the proposed MFL scenario.  The resource value was assumed to be protected if the frequency of occurrence associated with the withdrawal scenario did not differ unacceptably from the baseline condition.  The HSW evaluation of specific indicators of protection and associated stage frequency, under the leadership of Dr. Ken Watson and with the assistance of Dr. Scott Emery, Dr. Donald Carpenter, Mr. Dean Mades, Dr. Lei Yang, and Ms. Cindy Lee Westergard draws heavily on recent work by the District although numerous other technical references were utilized as well.

Specific indicators for protection were evaluated for each of the ten WRVs, and potential changes in return intervals were quantified for eight WRVs.  A total of 86 specific indicators were evaluated for eight water resource values, 58 of which are associated with WRV-2, Fish and Wildlife Habitat and the Passage of Fish.  The evaluations of indictors for Maintenance of Freshwater Storage and Supply lead to the conclusion by deduction that there would be no appreciable or measurable change from baseline conditions, thus the proposed MFLs and withdrawal scenario would be protective of the WRV.  An evaluation of Estuarine Resources was not performed, and HSW relied solely on the work by others and on downstream sections of the river.  Our opinion is that the proposed MFLs regime for the section of Lake Poinsett of the St. Johns River is protective of the ten water resource values under a wide range of flow conditions.  The protection of WRV-2 would be marginalized at high flows and stages by the proposed withdrawal scenario to the extent that vegetation transect monitoring should continue.  In addition to vegetation monitoring, a site-specific study of minimum flow velocities conducive to the success of the American shad is recommended, although the specific indicators evaluated for this species demonstrate protection of this value relative to baseline conditions.