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

Chapter 3 Goals, Ecological Resources of Concern and
Key Habitat Indicators
3.1 Goal – Preventing Significant Harm
The goal of a MFLs 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." What constitutes "significant harm" was not
defined. 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), 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
a 15% change in habitat availability as a measure of unacceptable loss, it is
important to note that percentage changes employed for other instream flow
determinations have ranged from 10% to 33%. For example, Dunbar et al.
(1998), in reference to the use of PHABSIM, noted, "an alternative approach is to
select the flow giving 80% habitat exceedance percentile," which is equivalent to
a 20% decrease. Jowett (1993) used a guideline of one-third loss (i.e., retention
of two-thirds) of existing habitat at naturally occurring low flows, but
acknowledged that "[n]o methodology exists for the selection of a percentage
loss of "natural" habitat which would be considered acceptable."
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