Alafia River Minimum Flows and Levels - Southwest Florida Water ...

Chapter 2 Basin Description with Emphasis on Land Use,
Hydrology and Water Quality
This chapter includes a brief description of the Alafia River watershed and is
followed by a presentation and discussion of land use, hydrology, and water quality
data relevant to the development of MFLs on the freshwater segment of the Alafia
River. Land use changes within the basin are discussed to set the stage for a
hydrology discussion that follows and to address questions that have been raised
regarding the potential impact of land use changes on river flow volumes (see SDI
2003). Water chemistry changes are discussed to illustrate how land use changes
associated with phosphate mining have played a significant role in observed trends
in certain water quality parameters, and to demonstrate how these trends are useful
in interpreting flow changes through time.
With respect to hydrology and trends in flow, a number of important observations
and conclusions are made that affect how MFLs are developed not only for the
Alafia River but for flowing systems in general. Concern over apparent declining
flow trends is an issue not only for the Alafia River, but for essentially all rivers in the
SWFWMD for which MFLs will be developed. Before discussing trends in Alafia
River flow, some discussion is devoted to river flow patterns and trends throughout
the SWFWMD and Florida. This is important, since it is concluded that there is a
significant climatic factor that must be considered when developing MFLs. It is
argued that some flow trends that should be expected as a result of a natural climate
oscillation have been interpreted as primarily anthropogenic (refer to Kelly 2004). In
addition, it is demonstrated that there are at least two potential benchmark periods
that should be considered when developing MFLs, and that selection of the proper
benchmark period is a critical part of the process. A "Building Block" approach to
establishing MFLs (as suggested in the peer review of the upper Peace River MFL –
Gore et al. 2002) is developed based on the inherent seasonal hydrologic pattern of
rivers in the SWFWMD. Further it is argued that the "percent of flow" approach to
permitting surface water withdrawals has many desirable attributes. Biological
connections with hydrology are addressed in Chapters 4 and 5, and result in MFL
recommendations specific to the freshwater segment of the Alafia River.
2.1 Watershed Description
2.1.1 Geographic Location
The Alafia River watershed extends over parts of two counties (Figure 2-1) including
much of the eastern and southern portions of Hillsborough County, and a smaller
portion of west-central Polk County. It is bounded on the north by the Hillsborough
2-1