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

Chapter 3 Ecological Resources of Concern and Key
Habitat Indicators
"There is no universally accepted method or combination of methods that
is appropriate for establishing instream flow regimes on all rivers or
streams. Rather, the combination or adaptation of methods should be
determined on a case-by-case basis; . . . In a sense, there are few bad
methods – only improper applications of methods. In fact, most . . .
assessment tools . . . can afford adequate instream flow protection for all
of a river's needs when they are used in conjunction with other
techniques in ways that provide reasonable answers to specific questions
asked for individual rivers and river segments. Therefore, whether a
particular method 'works' is not based on its acceptance by all parties but
whether it is based on sound science, basic ecological principles, and
documented logic that address a specific need" (Instream Flow Council
2002).
3.1 Goal – Preventing Significant Harm
The goal of an MFL determination is to protect the resource from significant harm
due to withdrawals and was broadly defined in the enacting legislation as "the
limit at which further withdrawals would be significantly harmful to the water
resources or ecology of the area." The District has identified loss of flows
associated with fish passage and maximization of stream bottom habitat with the
least amount of flow as significantly harmful to river ecosystems. Also, based
upon consideration of a recommendation of the peer review panel for the upper
Peace River MFLs (Gore et al. 2002), we propose that significant harm in many
cases can be defined as quantifiable reductions in habitat.
In their peer review report on the upper Peace River, Gore et al. (2002) stated,
"[i]n general, instream flow analysts consider a loss of more than 15% habitat, as
compared to undisturbed or current conditions, to be a significant impact on that
population or assemblage." This recommendation was made in consideration of
employing the Physical Habitat Simulation Model (PHABSIM) for analyzing flow,
water depth and substrate preferences that define aquatic species habitats.
With some exceptions (e.g., loss of fish passage or wetted perimeter inflection
point), there are few "bright lines" which can be relied upon to judge when
"significant harm" occurs. Rather loss of habitat in many cases occurs
incrementally as flows decline, often without a clear inflection point or threshold.
Based on Gore et al. (2002) comments regarding significant impacts of habitat
loss, we recommend use of a 15% change in habitat availability as a measure of
significant harm for the purpose of MFLs development. Although we recommend
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