Chapter 1 Minimum Flows and Levels - Southwest Florida Water ...

Fundamental to the approach used for development of minimum flows and levels is the
realization that a flow regime is necessary to protect the ecology of the river system.
The initial step in this process requires an understanding of historic and current flow
conditions to determine if current flows reflect past conditions. If this is the case, the
development of minimum flows and levels becomes a question of what can be allowed
in terms of withdrawals before significant harm occurs. If there have been changes to
the flow regime of a river, these must be assessed to determine if significant harm has
already occurred. If significant harm has occurred, recovery becomes an issue. The
District has adopted an approach for establishing benchmark flow periods that involves
consideration of the effects of multidecadal climatic oscillations on river flow patterns.
The approach, which led to identification of separate benchmark periods for flow
records collected prior to and after 1970, was used for development of MFLs for the
freshwater segment of the Alafia River, middle Peace River, and the Myakka River
(Kelly et al. 2005a, Kelly et al. 2005b, Kelly et al. 2005c). This is not the case on the
Braden River where the period of record on the longest term gage site date back only to
the late nineteen-eighties.
Following assessment of historic and current flow regimes and the factors that have
affected their development, the District develops protection standard statistics or criteria
for preventing significant harm to the water resource. For the upper segment of the
Peace River, criteria associated with fish passage in the river channel and maximization
of the wetted perimeter were used to recommend a minimum low flow (SWFWMD
2002). Criteria associated with medium and higher flows that result in the inundation of
woody habitats associated with the river channel and vegetative communities on the
floodplain were described. These criteria were not, however, used to develop
recommended levels, due to an inability to separate water withdrawal impacts on river
flow from those associated with structural alterations within the watershed. For the
middle segment of the Peace River, Alafia River, and the upper segment of the Myakka
River, the District has used fish passage, wetted perimeter and other criteria to protect
low flows and applied approaches associated with development of medium to high flow
criteria per recommendations contained in the peer review of the proposed upper Peace
River minimum flows (Gore et al. 2002). These efforts have included collection and
analyses of in-stream fish and macroinvertebrate habitat data using the Physical Habitat
Simulation (PHABSIM) model, and evaluation of inundation characteristics of floodplain
habitats.
1.5.1 A Building Block Approach
The peer-review report on proposed MFLs for the upper segment of the Peace River
(Gore et al. 2002) identified a "building block" approach as "a way to more closely mirror
original hydrologic and hydroperiodic conditions in the basin". Development of
regulatory flow requirements using this type of approach typically involves description of
the natural flow regime, identification of building blocks associated with flow needs for
ecosystem specific functions, biological assemblages or populations, and assembly of
1-6