Establishment of Minimum Flows and Levels for the Everglades: Project Status and
Future Directions
Catherine McCarthy, David Swift and Joel VanArman
South Florida Water Management District, West Palm Beach, Florida
The Everglades Minimum Flow and Level (MFL) project, which established minimum
flows and levels for the five Water Conservation Areas (WCAs), the Holey Land and
Rotenberger Wildlife Management Areas and the freshwater portions of Everglades
National Park (ENP), commenced in 1996. The Technical Documentation to support
MFL development was published in February 2000, and the final rule was adopted by the
South Florida Water Management District Governing Board in September 2001. State
Law defines the minimum level as the “…level of groundwater within any aquifer and
the level of surface water at which further withdrawals would be significantly harmful to
the water resources of the area…” (Section 373.042 (1) Florida Statutes). Significant
harm is defined as the temporary loss of water resource functions, which result from a
change in surface or ground water hydrology, that takes more than two years to recover,
but which is considered less severe than serious harm. The minimum level criteria and
definition of significant harm for the Everglades were based on protecting the following
six water resource functions: 1) providing hydropatterns that will support Everglades
ecosystems; 2) preventing saltwater intrusion of the Biscayne aquifer; 3) providing
natural biological filtering and nutrient cycling; 4) providing refugia for aquatic wildlife;
5) preventing invasion by undesirable species; and 6) maintaining desired salinities in
coastal estuaries. The proposed minimum water level criteria focused on protection of
the two dominant hydric soil types (peat and marl) found within the Everglades as
follows:
Water levels within wetlands overlying organic peat soils within the WCAs, Rotenberger
and Holey Land Wildlife Management Areas, and Shark River Slough should not fall 1.0
feet or more below ground level for more than 30 days duration, at specific return
frequencies, as defined for 15 gauge locations shown in Table 1.
Water levels within marl-forming wetlands that are located east and west of Shark River
Slough, the Rocky Glades, and Taylor Slough within Everglades National Park should
not fall more than 1.5 feet below ground level for more than 90 days duration, at specific
return frequencies, as defined for 4 gauge locations shown in Table 1.
The final Everglades MFL Rule language included the stipulation that additional wetland
research would be conducted to confirm or refine the MFL return frequency criteria that
will not cause significant harm to marl-forming wetland plant and animal communities.
The return frequencies defined for marl soils represented the expert opinion of District
staff based on management targets developed in the Comprehensive Everglades
Restoration Plan (CERP) and Lower East Coast (LEC) planning processes, and output of
the Natural System Model version 4.5 F (NSM v 4.5 Final). It was the expert opinion of
Everglades National Park staff that NSM v 4.5 Final may not properly simulate
hydrologic conditions within the Rockland Marl marsh and Taylor Slough and
Table 1. Minimum Water Levels, Duration, and Return Frequencies for Key Water Management
Gauges Located Within the Remaining Everglades.
Minimum Depth
Return
Key
Indicator Soil (ft) and Duration Frequency
Area
Gauge
Region Type
(days)
(years)
Water Conservation Areas
WCA-1
1-7
27
Peat -1.0 ft >30 days
1-in-4
WCA-2A
2A-17
24
Peat -1.0 ft >30 days
1-in-4
WCA-2B
2B-21
23
Peat -1.0 ft >30 days
1-in-3
WCA-3A North
3A-NE
21
Peat -1.0 ft >30 days
1-in-2
WCA-3A North
3A-NW
22
Peat -1.0 ft >30 days
1-in-4
WCA-3A North
3A-2
20
Peat -1.0 ft >30 days
1-in-4
WCA-3A North
3A-3
68
Peat -1.0 ft >30 days
1-in-3
WCA-3A Central
3A-4
17
Peat -1.0 ft >30 days
1-in-4
WCA-3A South
3A-28
14
Peat -1.0 ft >30 days
1-in-4
WCA-3B
3B-SE
16
Peat -1.0 ft >30 days
1-in-7
Everglades Agricultural Area
Rotenberger WMA
Rotts
28
Peat -1.0 ft >30 days
1-in-2
Holey Land WMA
HoleyG
29
Peat -1.0 ft >30 days
1-in-3
Everglades National Park
NE Shark River Slough
NESRS-2
11
Peat -1.0 ft >30 days
1-in-10
Central Shark River Slough
NP-33
10
Peat -1.0 ft >30 days
1-in-10
Central Shark River Slough
NP 36
9
Peat -1.0 ft >30 days
1-in-7
Marl wetlands east of Shark
NP-38
70
Marl -1.5 ft >90 days
1-in-3
River Slough
Marl wetlands west of Shark NP-201
12
Marl -1.5 ft >90 days
1-in-5
River Slough
G-620
Rockland Marl marsh
G-1502
8
Marl -1.5 ft >90 days
1-in-2
Taylor Slough
NP-67
1
Marl -1.5 ft >90 days
1-in-2
that the proposed return frequencies defined by District staff may not necessarily protect
these marl-forming wetlands from significant harm.
A literature review is being conducted by District staff to summarize the results of recent
research efforts by scientists at various universities, the United States Geological Survey,
and ENP to better understand marl forming wetland ecosystems and the water level
regimes required to prevent significant harm of the resource. The literature that has been
reviewed so far, includes long-term vegetation studies that examine the species
composition of sites with varying hydroperiods; macroinvertebrate and small fish studies,
which investigate how species composition, abundance, distribution, growth rate, overall
size, dormancy patterns, dispersal patterns and survivorship vary between short,
intermediate and long hydroperiod sites; and studies that examine the interactions of the
aquatic-animal community with the geologic structure and hydrologic elements of the
Rocky Glades.
District staff are also analyzing recent water level data from each key gauge listed in
Table 1 to document the water level conditions experienced within the various indicator
regions and to determine the duration and magnitude of MFL exceedances that have
occurred in recent years, including the 2000-2001 regional drought. The results of these
analyses will be compared to the CERP Adaptive Management Program (Restoration,
Coordination and Verification - RECOVER) performance measures to serve as
background measurements of the water level conditions prior to the implementation of
various CERP Projects. Elements of the CERP and the LEC Regional Water Supply Plan
(LEC Plan) constitute the District’s MFL Recovery Plan, which is required for water
bodies that are not expected to meet the MFL criteria as per Chapter 373.042 (1) F.S.
In 2005, all of the MFL criteria set for the five Water Conservation Areas, the Holey
Land and Rotenberger Wildlife Management Areas and the freshwater portions of
Everglades National Park will be reviewed in conjunction with the update of the LEC
Plan. The MFL technical documentation will be revised, if necessary, to incorporate
changes required by additional research, changes in CERP projects or components, and
the results of ongoing monitoring efforts. Furthermore, the minimum level, duration,
and return frequency components defined for the marl marsh habitat will be compared to
the updated version of NSM and the performance measures that are being developed for
RECOVER, once these work products become available.
Catherine McCarthy, South Florida Water Management District, 3301 Gun Club Road,
West Palm Beach, FL 33406, Phone: 561-682-6325, Fax: 561-681-6264,
cmccart@sfwmd.gov