The St. Johns River is the longest river in Florida, containing 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 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, including the potential withdrawal of 262 million gallons per day from the St. Johns River. To more comprehensively evaluate the environmental impacts of withdrawing this water from the river, the District embarked on a two-year Water Supply Impact Study (WSIS), and requested the involvement of the National Research Council. The present volume reviews the Phase I work of the WSIS and provides recommendations for improving Phase II.