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

Discharge, on the other hand, refers to the volume of water moving past a point, and
depending on the size of the stream (cross sectional area), similar volumes of water
can be moved with quite large differences in the rate of flow (velocity). The volume
of water moved through a stream can be particularly important to an estuary. It is
the volume of freshwater that mixes with salt water that determines what the salinity
in a fixed area of an estuary will be. This is especially important for organisms that
require a certain range of salinity. The volumes of fresh and marine water determine
salinity, not the flow rate per se; therefore, volume rather than flow is the important
variable to these biota. For the purpose of developing and evaluating minimum
flows, the District identifies discharge in cubic feet per second for field-sampling sites
and specific streamflow gaging stations.
In some cases, the water level or the elevation of the water above a certain point is
the critical issue to dependent biota. For example, the wetland fringing a stream
channel is dependent on a certain hydroperiod or seasonal pattern of inundation.
On average, the associated wetland requires a certain level and frequency of
inundation. Water level and the duration that it is maintained will determine to a
large degree the types of vegetation that can occur in an area. Flow and volume are
not the critical criteria that need to be met, but rather elevation or level.
There is a distinction between volumes, levels and flows that should be appreciated.
Although levels can be related to flows and volumes in a given stream (stream
gaging, in fact, depends on the relationship between stream stage or level and
discharge), the relationship varies between streams and as one progresses from
upstream to downstream in the same system. Because relationships can be
empirically determined between levels, flows and volumes, it is possible to speak in
terms of, for example, minimum flows; however, one needs to appreciate that
individual species and many physical features may be most dependent on a given
flow, level or volume or some combination of the three for their continued survival or
occurrence. The resultant ecosystem is dependent on all three.
1.7 Content of Remaining Chapters
In this chapter, we have summarized the requirements and rationale for developing
minimum flows and levels in general and introduced the need for protection of the
flow regime rather than protection of a single minimum flow. The remainder of this
document considers the development of minimum flows and levels specific to the
freshwater segment of the Alafia River. In Chapter 2, we provide a description of the
basin and its hydrogeologic setting, and consider historic and current river flows and
the factors that have influenced the flow regimes. Identification of at least two
benchmark periods of flow, resulting from naturally occurring climatic oscillations is
noted, and seasonal blocks corresponding to low, medium and high flows are
identified. Water quality changes related to flow are also summarized in Chapter 2
1-9