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

Common to this list and the flow requirements identified by Hill et al. (1991) is the
recognition that in-stream flows and out of bank flows are important for ecosystem
functioning, and that seasonal variability of flows should be maintained. Based on these
concepts, the preconception that minimum flows (and levels) are a single value or the
absolute minimum required to maintain ecologic health in most systems has been
abandoned in recognition of the important ecologic and hydrologic functions of streams
and rivers that are maintained by a range of flows. And while the term “minimum flows”
is still used, the concept has evolved to one that recognizes the need to maintain a
“minimum flow regime”. In Florida, for example, the St. Johns River Water Management
District typically develops multiple flow requirements when establishing minimum flows
and levels (Chapter 40-C8, F.A.C) and for the Wekiva River noted that, “[s]etting
multiple minimum levels and flows, rather than a single minimum level and flow,
recognizes that lotic [running water] systems are inherently dynamic” (Hupalo et al.
1994). Also, in 2005, changes that acknowledge the importance of retaining the
hydrologic regime were made to the Florida Administrative Code. Specifically, Chapter
62-40.473(2) of the State Water Resources Implementation Rule currently directs that
"minimum flows and levels should be expressed as multiple flows or levels defining a
minimum hydrologic regime". This change was intended to protect variation in water
flows and levels that contributes to significant functions of ecosystems.
1.4 Ecosystem Integrity and Significant Harm
“A goal of ecosystem management is to sustain ecosystem integrity by protecting native
biodiversity and the ecological (and evolutionary) processes that create and maintain
that diversity. Faced with the complexity inherent in natural systems, achieving that
goal will require that resource managers explicitly describe desired ecosystem structure,
function, and variability; characterize differences between current and desired
conditions; define ecologically meaningful and measurable indicators that can mark
progress toward ecosystem management and restoration goals; and incorporate
adaptive strategies into resource management plans” (Richter et al. 1996). Although it
is clear that multiple flows are needed to maintain the ecological systems that
encompass streams, riparian zones and valleys, much of the fundamental research
needed to quantify the ecological links between the instream and out of bank resources,
because of expense and complexity, remains to be done. This research is needed to
develop more refined methodologies, and will require a multi-disciplinary approach
involving hydrologists, geomorphologists, aquatic and terrestrial biologists, and
botanists (Hill et al. 1991).
To justify adoption of a minimum flow for purposes of maintaining ecologic integrity, it is
necessary to demonstrate with site-specific information the ecological effects associated
with flow alterations and to also identify thresholds for determining whether these effects
constitute significant harm. As described in Florida’s legislative requirement to develop
minimum flows, the minimum flow is to prevent “significant harm” to the state’s rivers
and streams. Not only must “significant harm” be defined so that it can be measured, it
is also implicit that some deviation from the purely natural or existing long-term
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