The St. Johns River is the longest river in Florida, flowing 310 miles from Indian River County north to the Atlantic Ocean. The river drops only 30 feet from its headwaters to its mouth, such that the river has extensive freshwater wetlands, numerous large lakes, a wide estuarine channel, and a correspondingly diverse array of native flora and fauna. Water resource management in the river’s watershed, which accounts for 23 percent of Florida’s land area, is the responsibility of the St. Johns River Water Management District (the District). The District must provide water for the region’s 4.4 million residents as well as numerous industrial and agricultural users, all while protecting natural systems within the river basin.

With population growth in the watershed expected to surpass 7.2 million in 2030, the District, through its water resources planning process, has begun to identify alternative sources of water beyond its traditional groundwater sources. Water reuse, desalination, and new surface water supplies are all under consideration, including the potential withdrawal of 262 million gallons per day (MGD) from the St. Johns River. To more comprehensively evaluate the environmental impacts of withdrawing this water from the river, in early 2008 the District embarked on a two-year Water Supply Impact Study (WSIS). Later that year, the District requested the involvement of the National Research Council (NRC) to review scientific aspects of the study and provide advice to its ongoing effort (see Chapter 1 for the NRC committee’s statement of task). This first report of the committee reviews the Phase I work of the WSIS and provides recommendations for improving Phase II. The report is organized along the lines of the seven scientific workgroups of the District, which include hydrologic and hydrodynamic modeling, biogeochemistry, plankton and water quality, benthos, the littoral zone, fish, and wetlands and wetland-dependent species.

It should be noted that the WSIS does not consider the largest tributary to the St. Johns, the Ocklawaha River, to the same degree as the St. Johns River itself because (1) the Ocklawaha is hydrologically distinct from the St. Johns, (2) a focused study on the St. Johns River was more feasible given time and resource constraints, and (3) a separate analysis of minimum flow and level requirements for the Ocklawaha River is planned for the near future. Additionally, the WSIS does not consider the impacts of population growth that an additional water withdrawal of 262 MGD could support, as the District has no direct authority over growth and associated land use changes in the basin.