Report SFRC-88/01 

Hydrologic Effects of the 1984 

through 1986 L-31 Canal Drawdowns 

i on the Northern Taylor Slough Basin 

of Everglades National Park 

Everglades National Park, South Florida Research Center, P.0. Box 279, Homestead, Florida 33030 \ 
Hydrologic Effects of the 1984 through 1986 

G31 Canal Drawdowns on the 

Northern Taylor Slough Basin 

of Everglades National Park 

Report SFRC-88/0 1 

Robert A. Johnson, Joel I. Wagner, 

Deborah J. Grigsby, and Virginia A. Stern 

South Florida Research Center 

Everglades National Park 

Homestead, Florida 
Johnson, Robert A, Joel I. Wagner. Deborah J. Grigsby, and Virginia A. Stern. 1988. 

Hydrologic Effects of the 1984 through 1986 G31 Canal Drawdowns on the 

Northern Taylor Slough Basin of Everglades National Park. South Florida Research 

Centcr Report SFRC-88/01. 45 p. 
TABLEOFCONTENTS 

PAGE 

LIST OF TABLES 

LIST OF FIGURES 

LIST OF APPENDICES 

ACKNOWLEDGEMENTS 

EXECUTIVE SUMMARY 

INTRODUCTION 

STUDY AREA 

Taylor Slough Headwaters 

Upper Taylor Slough 

Middle Taylor Slough 

The Frog Pond Agricultural Area 

TAYLOR SLOUGH HYDROLOGY 

Rainfall 

Surface Water Hydrology 

Groundwater Hydrology 

THE DADE COUNTY WATER MANAGEMENT SYSTEM 

The Central and Southern Florida Project 

The South Dade Conveyance System 

The L31W Canal System 

TAYLOR SLOUGH/FROG POND WATER MANAGEMENT 

L-3 1W Design Stages and 02erational Changes 

The Frog Pond Experimental Drawdown Agreement 

STUDY METHODS 

The Field Monitoring Network 

Techniques for Documenting Drawdown Effects 

Drawdown Responses at Continuous Stage Recorders 

Determination of Background Recession Rates 

Development of Water Level Contour Maps 

RESULTS AND DISCUSSION 

Hydrologic Conditions and Structure Operations during 1984 

1984 Drawdown Effects at Continuous Stage Recorders 

Hydrologic Conditions and Structure Operations during 1985 

1985 Drawdown Effects ht Continuous Stage Recorders 

Hydrologic Conditions and Structure Operations during 1986 

1986 Drawdown Effects at Continuous Stage Recorders 

Summary of Water Level Declines 
PAGE 

Water Level Contouring Results 

1984 Contour Results 

1985 Contour Results 

The 1984 through 1986 Canal Drawdown Operations in Relation 

to Operations During Prior Periods 

SUMMARY AND CONCLUSIONS 

LITERATURE CITED 

APPENDICES 
LIST OF TABLES 

PAGE 

1. Summary of Royal Palm Ranger Station Rainfall Conditions 

2. Summary of Initial Design Settings and Operational Changes at 

Structures S- 174 and S- 175 

3. Calculated Background Recession Rates for Station R3 1 10 

4. Summary of Observed Water Level Declines During the 1984 through 

1986 Study Periods 

5. Dates at which L31W Stages Receded to 3.50 feet 
LIST OF FIGURES 

FIGURE PAGE 

1. Map of the Taylor Slough Drainage Basin 

2. The Dade Counw Water Managrnent System 

3. Map of the Northern Subarea 

4. Map of the Southern Subarea 

5. Hydrologic Conditions and Structure Operations During 1984 

6. 1984 Drawdown Effects at Co~tinuous Stage Recorders 

7. Hydrologic Conditions and Structure Operations During 1985 

8. 1985 Drawdown Effects at Continuous Stage Recorders 

9. Hydrologic Conditions and Structure Operations During 1986 

10. 1986 Drawdown Effects at Continuous Stage Recorders 
LIST OF APPENDICES 

APPENDIX 

Appendix A. The 1984 "Frog Pond Drawdown Agreement 

Appendix B. Stations Included in the Study 

Appendix C. Water Level Contour Maps (1984-85) 
ACKNOWLEDGEMENTS 

The authors would like to thank a number of individuals who contributed to the review 

of this report. Louis Hornung and James Vearil of the U.S. Army Corps of Engineers 

provided much assistance with background on project authorization and design criteria 

for the water control structures and conveyance canals in the study area. Thomas 

MacVicar of the South Florida Water Management Distfict assisted with documentation 

of current operational practices and the history of operational changes in the L-31W 

and C-111 canals as well as providing a critical review of the overall document. Rod 

Ghiotto, representing the farming interests in the Frog Pond. provided a thorough 

review of the entire report and helped to focus the multiple objectives of the study. 
EXECUTIVE SUMMARY 

A large portion of the headwaters region of Taylor Slough lies within the lower East 

Everglades and an adjacent agricultural area referred to as the "Frog Pond. For this 

reason. the management of water levels in these areas is critical to the water resources 

of the basin. In early 1984. Frog Pond area farmers requested that a plan be developed 

to lower water levels in the area allowing land preparation and planting to begin in midOctober. An experimental one-year agreement was negotiated between the National 

Park Service and the Frog Pond farmers to lower water levels to 3.50 feet by October 15 

in the L31W and C- 11 1 canals adjacent to the Frog Pond. This one-year experiment 

was undertaken to evaluate the effects of the drawdown on the water resources of 

Taylor Slough. The canal drawdown operations were continued through the 1985-86, 

1986-87, and 1987-88 growing seasons, in conjunction with water delivery 

experiments in the Shark Slough basin. This experimental test was established by an 

agreement between the South Florida Water Management District and a group of south 

Dade farmers. 

This report documents the results of hydrologic investigations conducted by the 

National Park Service. evaluating the impacts of 1984 through 1986 L31W canal 

drawdoh operations on the water resources of the northern Taylor Slough basin of 

Everglades National Park. Drawdown impacts on the hydrologic conditions in the 

central and southern portions of the Taylor Slough Basin were not investigated. The 

1984 drawdown period began on the morning of October 15. Control structure S-175 

remained open for seven days to stabilize L31W canal stages at the required 3.50 feet. 

During the first 24 hours, S-175 headwater stage dropped 0.58 feet. Maximum water 

level declines of 0.42 feet occurred in the adjacent Taylor Slough wetlands over the 

same period. 

In 1985. rainfall and upstream water management operations associated with the 

Shark Slough experimental agreement prompted nearly continuous use of the L31W 

control structures. The drawdown began on the afternoon of October 10. 1985, and 

lasted approximately five days. Between the 1 lth and the 16th. S-175 headwater stage 

dropped 0.82 feet. Maximum water level declines of 0.71 feet occurred in the Taylor 

Slough wetlands over the same five-day period. Rainfall between October 16 and the 

19th raised L31W canal stage above the growing season regulation level prompting a 

second drawdown. Structure S-175 remained open until November 6 to maintain the 

growing season regulation stage. 

During the 1986 study period, headwater stage at S-175 dropped below the growing 

season regulation level prior to October 15 following a long period of below normal 

rainfall. Rainfall during the evening of October 15 raised canal stages above the 

growing season regulation level prompting a 24-hour period of S-175 operation. 

Between October 16 and the 17th. S-175 headwater stage dropped 0.55 feet. Maximum 

water level declines of 0.39 feet occurred in the adjacent Taylor Slough wetlands over 

the same period. 

Water level recessions in the L31W canal below 3.50 feet by October 15 were almost 

unprecedented prior to the 1984 experimental agreement occurring only once during 

the previous 14 years. The average date of water level recessions to 3.50 feet under 

previous water management operations was November 20, five weeks later than the 

date of the 1984 experimental drawdown. Prior to the 1984 drawdown experiment, 
date of the 1984 experimental drawdown. Prior to the 1984 drawdown experiment, 

there was no formal agreement to use the L31W canal to provide additional flood 

protection below the established 4.50-foot operational setting. The hydrologic 

investigations conducted during the 1984 through 1986 study periods indicate that the 

L31W canal agricultural drawdown operations produced water level reductions 

throughout northern Taylor Slough, particularly in the areas adjacent to the G31W 

canal. All of these drawdowns occurred during years when the total annual rainfall was 

over 10 inches below normal and monthly total rainfall during the September through 

November study periods was also below normal (Table 1). 

The L31W borrow canal was designed "to convey flow to replenish the fresh water 

supply in the Taylor Slough area of the park" (U.S. Army Corps of Engineers 1967). 

Structure S-175 was originally designed to "maintain water levels up to 5.00 feet and 

provide a means of either diverting borrow canal discharge overland into Taylor Slough 

or passing it south to be distributed overland from the borrow canal south of State 

Road 27" into central Taylor Slough (U.S. Army Corps of Engineers 1963). Following 

this initial design study. S-175 headwater stage was reduced by 0.50 feet and set to 

maintain upstream optimum water levels of up to 4.50 feet for as long as local rainfall 

remained adequate (U.S. Army Corps of Engineers 1973). 

The overall results of this study indicate that management of wet season stages below 

4.50 feet in the upper portion of the L3 1W canal causes drainage of the adjacent Taylor 

Slough wetlands. Review of water level data collected during the last four and one-half 

years of the Frog Pond drawdown shdy indicate that during the months of June 

through November, L31W canal stages generally remained well below the stage in the 

Taylor Slough wetlands. Under these conditions, the L-31W canal acts as a 

groundwater sink draining the adjacent wetlands. This is inconsistent with the initial 

design purpose of the L-31W canal which states that the canal should replenish the 

fresh water supply to the Taylor Slough basin. The lowering of L31W canal stages to 

allow earlier land preparation and planting in the Frog Pond agricultural area further 

exacerbates the drainage effects of the canal system and is incompatible with the 

system design intent of restoring fresh water flows to the Taylor Slough basin. 
INTRODUCTION 

On June 4. 1984. Everglades National Park and the individuals and corporations 

fanning in the "Frog Pond agricultural area reached an agreement outlining an 

experimental drawdown in the L31W and C-111 canals for the 1984-85 growing 

season. The purpose of the experiment was to determine the hydrologic effects of 

lowering canal stages in the Frog Pond area which had been requested by the farmers to 

permit earlier planting of seasonal row crops. In a letter dated June 22, '1984. the 

executive director of the South Florida Water Management District stated that the 

District considered the agreement acceptable and would honor its terms within the 

context of concurrent flood control and water supply commitments. The experiment 

began in October 1984 and continued through the 1984-85 growing season as planned. 

Following the experiment, the canal drawdown operations were extended through the 

1985-86, 1986-87. and 1987-88 growing seasons in conjunction with Everglades 

National Park water delivery experiments authorized by Public Laws 98- 18 1 and 99- 190 

(U.S. Congress 1983. 1985). 

The purpose of this document is to report the results of hydrologic investigations 

undertaken by Everglades National Park to evaluate the effects of the 1984 through 

1986 L31W canal drawdown operations on the water resQurces of the northern Taylor 

Slough basin in Everglades National Park. The general goals of the hydrologic 

investigations included: 

1. Determination of the water level recession rates. response times. and 

maximum water level declines attributable to the agricultural canal drawdowns as well 

as the areal extent of these impacts in the northein Taylor Slough basin. 

2. Comparison of L31W canal management during the 1984 through 1986 

drawdown periods with design stages and pre-drawdown management practices. 

STUDY AREA 

Taylor Slough, a 158 square mile freshwater wetland. extends more than 20 miles from 

its headwaters north of the Frog Pond agricultural area to the coastal mangrove fringe 

along Florida Bay (Fig. 1). The basin is situated along the northeastern boundary of 

Everglades National Park and is the area of the park that is in closest proximity to 

nearby agricultural activities. The Taylor Slough basin has been divided into five 

distinct physiographic zones by Schomer and Drew (1982). These zones include the 

headwaters, upper, middle, lower slough. and the coastal swamps and lagoons. The 

hydrologic investigations described in this report include only data collected from 

portions of the headwaters. upper. and middle slough areas of the basin. 

Taylor Slou~h Headwaters 

The headwaters zone of Taylor Slough is bordered on the north by a narrow strip of land 

known locally as the Rocky Glades and extends southward to the point where the main 

channel of Taylor Slough intersects the L31W canal (location of the S-332 pump 

station). Most of the headwaters area lies outside of Everglades National Park in the 

southern portion of the East Everglades. The area is characterized by rocky. open. 

muhly prairies (Muhlenbergia fdpes) with thin eroded marl soils overlying a solution 

riddled limestone (surface. The hydroperiods in this area are short with surface 

inundations of generally less than two months (Schomer and Drew 1982). 
SCALE (miles) 

Figure 1. Map of the Taylor Slough Drainage Basin showing the study area, the 

Frog Pend agricultural area, and the G31W and C-111 canal systems. The dashed 

line represents the historical boundaries of Taylor Slough. Within the study area, 

the park boundary follows the G31W canal alignment. 
Umer Taylor Slough 

The upper portion of Taylor Slough extends from the intersection of the slough and the 

G31W canal southward nearly two miles below the main park road. The central portion 

of this zone maintains a distinct dry season flow channel because of S-332 pump 

station discharges. This wetter depression has hydroperiods generally exceeding nine 

months and is dominated by sawgrass (Cladturn jamaicensis) and spike rush (Eleocharis 

cellulosa) with small scattered willow stands (Salk Carolinians). The depression thins 

laterally into muhly prairies with scattered small hardwood hammocks on higher 

exposed limestone outcroppings. The muhly prairies of the upper portion of the basin 

have shallow marl soils with scattered limestone outcrops. These open prairies have 

intermediate hydroperiods with surface inundations of approximately two to four 

months (Rintz and Loope 1978). The upper zone of Taylor Slough forms a breach in the 

Coastal Ridge system separating the upland areas west of the L31W canal (Long Pine 

Key] from the major portion of the ridge system to the east of the slough (Schomer and 

Drew 1982). 

Middle Taylor Slough 

The middle portion of Taylor Slough extends from a point two miles south of the park 

road southward approximately four miles. At that point the main part of the slough is 

joined by a large area of extended hydroperiods which is supplemented by discharges 

from the southern end of the L31W canal. This middle portion of the slough has 

thicker marl soils overlying scattered peat substrates. This zone is dominated by large 

areas of mixed sawgrass and spike rush with smaller areas of dense sawgrass and 

willow stands. The middle slough zone has longer hydroperiods with surface 

inundations of approximately four to eight months (RLntz and Loope 1978). 

The Frog Pond &ricultutal Area 

The Frog Pond is a 5,000-acre agricultural area lying just east of the upper zone of 

Taylor Slough. The area is bordered on the east by the C-111 canal and on the north 

and west by the G31W canal (Fig. 1). The dashed line defining the predrainage borders 

of Taylor Slough was drawn from U. S. Soil Conservation Service soil survey maps 

(U.S.D.A. 1958) which showed the various phases of the Perrine Marl as wetland soils. 

Figure 1 indicates that a large portion of the Frog Pond area lies within the historic 

marsh. The remaining portions of the Frog Pond occur on the higher areas of the 

Coastal Ridge system. 

TAYLOR SLOUGH HYDROLOGY 

Rainfall 

The study area has a tropical savanna climate with a relatively long and intense dry 

season occurring from approximately November through April and a wet season 

occurring from approximately May through October. Dry season rainfall results from 

large scale winter frontal-storms which produce relatively uniform rainfall over a large 

Wea. Dry season rainfall contributes approximately 20 percent of the total annual 

precipitation. Wet season rainfall is primarily from local convective storms which 

produce variable rainfall over a small area. Wet season rainfall is usually intense and 

of a short duration with less frequent day-long storms associated with tropical 

disturbances. Wet season rainfall contributes approximately 80 percent of the total 

annual precipitation (Schromer and Drew 1982). 
There are a large number of rain gages throughout the study region with three stations 

(R3110. S-332, and Royal Palm) occurring within the upper portion of the Taylor Slough 

basin (Figs. 3 and 4. pp. 12-13). The rain gage at R3110 was not installed until 

September 1985 and did not provide reliable data for the first few months. For this 

reason, daily rainfall totals reported throughout this report were based on rainfall 

amounts recorded at the S-332 pump station in northern Taylor Slough. The rain gage 

at Royal Palm has the longest available record (37 years) and was used in this report for 

period of record comparisons. Monthly rainfall totals for the three study years and the 

period of record monthly means for the Royal Palm rain gage are summarized in Table 

1. For this rain gage, the annual, dry, and wet season period of record averages are 

56.9, 12.0, and 45.0 inches, respectively. 

Table 1. Summary of Royal Palm Ranger Station Rainfall 

Conditions During the Study Period and Over the 

Period of Record (1950 - 1987). 

PERIOD OF RECORD 1984 1985 1986 

MONTH MEAN ST. DEV. TOTAL TOTAL TOTAL 

January 

February 

March 

April 

May 

June 

July 

August 

September 

October 

November 

December 

ANNUAL 56.93 13.25 46.18 45.29 40.63 

Monthly and annual rainfall amounts are in inches. *(Indicates 

months with missing data during the study period.) 

Prior to the construction of the L31W canal system, surface water in the Taylor Slough 

basin was derived principally from local rainfall. Under high water conditions, 

sheetflow from the Shark Slough basin, north of the headwater area. contributed 

additional surface water to Taylor Slbugh. This water then flowed southward down the 

slough as sheetflow to Florida Bay or served to recharge the underlying aquifer. 

Suriace water discharge through the headwaters zone of the basin was initially 

measured as it passed through 80 culverts along a 7.5-mile stretch of Context Road 

north of the park boundary. These measurements were done by the U. S. Geological 

Survey beginning in 1976 and were discontinued in 1980. Surface water discharge into 

the upper zone of Taylor Slough is measured as it passes beneath a three mile stretch of 

the main park road known as the Taylor Slough Near Homestead flow section. These 

measurements began in August 1960 by the U. S. Geological Survey and were taken 

over in 1985 by Everglades National Park. Early studies by the U. S. Geological Survey 

(Earle and Hartwell 1973) showed that prior to the construction of the L31W canal 
mean annual discharge through the Taylor Slough Near Homestead flow section, hr the 

period from October 1960 through September 1968. was 34,300 acre-feet per year. For 

the same period, there was zero flow 46 percent of the time. Later studies by the 

National Park Service (Rose et al. 1981) showed that mean annual discharge through 

the Taylor Slough Near Homestead flow section, following construction of the L-31W 

canal, was 20,600 acre-feet per year over the period of October 1968 through 

September 1978. For the same period there was zero flow 52 percent of the time. 

Taylor Slough and the Frog Pond agricultural area overlie the Biscayne Aquifer. a highly 

permeable unconfined aquifer extending throughout southeastern Florida. 

Groundwater recharge occurs via rainfall and surface water infiltration as well as 

through seepage losses from the adjacent conveyance canals. Left undisturbed by 

canals, groundwater movement in the area follows local hydraulic gradients from the 

headwaters area of Taylor Slough toward Florida Bay. 

The Biscayne Aquifer is deflned by hydrogeologic properties rather than formational 

boundaries (Jarosewich and Wagner 1985) and is approximately 40 feet thick within the 

study area. Geologic well logs of a borehole drilled just north of structure S-175 

indicate that the upper portion of the aquifer consists of Miami Limestone with a total 

thickness of approximately eight feet. An oolitic facies of this unit forms surface 

exposures throughout the headwaters and upper portion of the bash. Below this is a 

thin limestone unit known locally as the Fort Thompson Formation. The Tamiami 

Formation underlies the Fort Thompson Formation. The base of the Biscayne Aquifer is 

defined as the bottom of the upper permeable limestone unit in the Tamiami Formation. 

Within the study area. the Fort Thompson Formation and the upper limestone unit of 

the Tamiami Formation have a total combined thickness of approximately 32 feet. 

Pump tests of deep boreholes in the study region give estimated transmissivities in the 

range of 700,000 to 1,000,000 feet squared per day and storage coefficients in the range 

of 0.20 to 0.25 with the most permeable and highest yielding zones within the Biscayne 

Aquifer. 

THE DADE COUNTY WATER MANAGEMENT SYSTEM 

The Central and Southern Florida Proiect 

The conveyance canals and control structures within Dade County are part of the 

Central and Southern Florida Project constructed by the Army Corps of Engineers for 

flood protection and water control purposes. The first phase of the project was 

authorized by the Flood Control Act of 1948 in accordance with the Comprehensive 

Report Plan (U.S. Congress 1948a). This first phase included most of the existing water 

control works in the Everglades Agricultural Area south of Lake Okeechobee along with 

the early flood control and drainage canals in the developed areas of the lower east 

coast. The remaining works of the Comprehensive Report Plan as well as other 

refinements and modifications were authorized by the Mood Control Acts of 1954, 1962. 

and 1968 (U.S. Congress 1954. 1962. and 1968). 

The first overall plan for flood protection and water control for southern Dade County 

was presented in the Survey Review Report on the Central and Southern Florida 

Project, South Dade County (U. S. Army Corps of Engineers 1961). The L31W canal 

system was not included as part of this plan. The remaining major flood control and 

water supply facilities for southern Dade County were addressed by the General Design 
Memorandum. South Dade County (U. S. Army Corps of Engineers 1963). The L31W 

canal and control structures S-174 and S-175 were added to the project as part of this 

memorandum following recommendations by the National Park Service and the Fish 

and Wildlife Service. 

The first major canal constructed in Dade County was the southern end of the Miami 

Canal, which was built in 1912 to drain the northern Everglades basin to the Atlantic 

coast (Fig. 2). In 1951 construction began on the L30 and the northern portion of the 

L31N canal and levee systems. These were built as part of the eastern protective levee 

system to protect the populated east coast from the Everglades during periods of 

flooding. 1d1961 construction began on the L29 canal and levee system which largely 

completed the closure of Water Consenration Area 3 (WCA 3) along the northern 

boundary of the park. At this time there was no connection between WCA 3 and the L 

30 and L3 1N canal systems so the canal systems south of the conservation areas could 

be used for flood protection but not for water supply. 

The earliest canal construction in southeastern Dade County began in 1955 with 

completion of the Snapper Creek canal (C-2) and later. in 1960. the Black Creek canal 

(-1) Both of these canals were initially constructed for drainage and were 

uncontrolled. During 1964 and 1965 a number of additional canals were added to the 

system (C- 100, C- 102. C- 103, and C- 11 1) and control structures were added to all of 

the new and existing canals to reduce saltwater intrusion. In 1966 construction began 

on the remainder of the L31N canal and structures S-173, S- 176. and S- 177 were 

added to control flows southward into the C-111 canal. At the same time several of the 

canals constructed earlier (C-1. C-102, and C-1031 were extended westward to the t 

31N canal to improve flood protection and allow for water supply to these basins. 

Construction of the L31W canal and structures S-174 and S-175 began in 1968 and 

was completed in early 1970. At this time the system still lacked sufficient control 

structures to move surplus water in WCA 3 to the lower east coast. 

The South Dade Conveyance System 

Concerns voiced by Everglades National Park that more freshwater was needed for 

Taylor Slough and the downstream areas of Florida Bay prompted Congress to 

authorize the construction of the south Dade Conveyance System as part of the Flood 

Control Act of 1968 (U.S: Congress 1968). This system of canals, control structures, 

and pumping stations was added to the Central and Southern Florida Project for the 

purposes of "conservation and conveyance of additional water supplies for Everglades 

National Park and for expanding agricultural and urban needs" along-the lower east 

coast (u.S. Congress 1968). The system was primarily designed to provide 55.000 acrefeet per year of supplemental water to the eastern portion of Everglades National Park to 

meet the congressionally mandated minimum delivery schedule which went into effect 

in October 1970. 

The first connection between Water Conservation Area 3A and the lower east coast 

canals occurred with the completion of structures S-333 and S-334 in the L29 canal in 

1978 (Fig. 2). These were installed to provide additional dry season water deliveries to 

the L3 1N canal. At this time the movement southward of this water was limited by the 

conveyance capabilities of the canal and structure S- 173. By 1983, the enlargement of 

the canal system. Completion of pump station S-33 1, and a new bridge over the G31N 

canal at Howard Drive removed this limitation. 
Figure 2. Map of Dade County showing the canals and water control structures in 

the general study area. The area below the 629 canal and lying between 631~ 

and the 667 extension canals is referred to as Northeast Shark Slough. This area 

is outside the boundaries of the Everglades National Park. 
The L31W Canal System 

The L-31W canal and its associated control structures were initially designed as a part 

of the east coast protective levee system. The L31W canal was designed for flood 

protection and to "convey flow to replenish the fresh water supply in the Taylor Slough 

area of the park." Conversely the levee was constructed to "prevent flooding from the 

Everglades National Park into agricultural and industrial areas lying to the east" [U. S. 

Army Corps of Engineers 1967). The canal and its associated levee are 11.1 miles long 

running from its confluence with the L31N canal southward to a point approximately 

2.4 miles south of the main park road (Fig. 1). Structure S-174 is located at the 

upstream end of the canal just west of its confluence with the L31N canal. The 

structure is an automatic gated spillway originally designed to pass a discharge of 500 

cubic feet per second (cfs) at a headwater stage of 6.00 feet and a tailwater stage of 5.50 

feet [all stages listed in this report are referenced to the National Geodetic Vertical 

Datum of 1929). Structure S-175 is located 1.4 miles north of the main park road. The 

structure is a set of three culverts with manually operated gates, originally designed to 

pass a discharge of 500 cfs at a headwater stage of 5.00 feet with a 0.50 foot head loss. 

The portion of the canal system downstream of structure S-175 is uncontrolled and 

stages fluctuate with the adjacent wetlands. 

TAYLOR SLOUGH/FROG POND WATER MANAGEMENT 

The Survey Review Report of the Central and Southern Florida Project for South Dade 

County (U. S. Army Corps of Engineers 1961) established the first plans for the water 

management system in the C- 11 1 canal east of the Frog Pond agricultural area. Later, 

the General Design Memorandum for South Dade County (U. S. Army Corps of 

Engineers 1963) added plans for the L31W canal system along the eastern park 

boundary in northern Taylor Slough. Provisions of these plans included flood 

protection for developed areas of southern Dade County and facilities to convey 

supplemental water to Taylor Slough to compensate for the estimated drainage effects of 

upstream canal systems on the basin's headwaters. 

Pump station S-332 was authorized by the Flood Control Act of 1968, as part of the 

South Dade Conveyance System, and was added to the L-31W canal to provide 

supplemental flows to upper Taylor Slough [U.S. Congress 1968). The 1968 

authorization specifically recognized provision of adequate water supplies to the 

Everglades National Park as a project purpose in order to preserve and restore its 

natural state and to maintain and protect its unique ecology. Construction began on 

the S-332 pump station in 1979 and it became operational by October 1980. Funds for 

construction were provided as part of the Monetary Authorization Act of 1970, which 

also establlshed the first monthly delivery schedule for this structure (U.S. Congress 

1970). This section was designed (1) to assure construction as soon as possible of the 

conveyance facilities necessary to deliver water to the drier eastern portions of the park 

and (2) to remove any uncertainty the amount of water the project is required to deliver 

to the park, by in effect amending. through this additional legislation, the project 

authorization. This pumping schedule was later revised by the "Agreement and Permit" 

for operation signed in 1976 by the Army Corps of Engineers, the South Florida Water 

Management District, and the National Park Service. The final approved pumping 

schedule provided 37.000 acre-feet per year to be delivered through the S-332 pump 

station. The approved minimum monthly pumpages were 740. 370, 185, 185, 370, 

6660. 7400, 2960, 5920. 7770. 3700. and 740 acre-feet for the months of January 

through December, respectively. 
t31W Design Stages and Operational Changes 

The first operations logs for control structures S- 174 and S- 175 began in August 1970. 

Structure S-174 remained closed and off until July 1971, when it was first set to 

operate on automatic. Prior to May 1979, S-174 was operated on automatic according 

to the established operations settings except during short periods of time when the 

structure was turned off due to low water levels. After May 1979. S-174 was routinely 

operated manually. and was opened at stages well below the established automatic gate 

settings. Structure S- 175 remained closed until September 1977. Prior to this date. L 

3 1W canal water levels were generally low with a mean stage of 2.79 feet at the S- 175 

headwater recorder. During this period, the few high canal stages that exceeded the S175 operations criteria were allowed to recede naturally. Between September 1977 and 

the start of the Frog Pond experiment. canal stages were higher with a mean stage of 

3.46 at the S-175 headwater recorder. During this period. S-175 was used routinely 

whenever canal stages exceeded the established operations criteria. In addition, S-175 

was operated at stages below the established criteria to lower stages for agricultural 

flood protection once the canal stage had receded naturally below approximately 3.50 

feet. 

Structure S-174 was originally designed to "maintain optimum water levels up to 6.00 

feet in the L31N borrow canal and discharge up to 500 cfs to the borrow canal of levee 

31W." Structure S-175 was originally designed to "maintain water levels up to 5.00 feet 

and provide a means of either divertlng borrow canal discharge overland into Taylor 

Slough or passing it south to be distributed overland from the borrow canal south of 

State Road 27" (U.S. Army Corps of Engineers 1963). Table 2 shows the original design 

stages and changes in the operating criteria for structures S-174 and S-175. 

Apparently, the first operations setting for S- 175 had been lowered to 4.50 feet following 

the completion of the Detailed Design Memorandum in 1967 (personal communications, 

USACOE and the SFWMD). In 1973, a General Design Memorandum was released 

describing the South Dade Conveyance System improvements (U.S. Army Corps of 

Engineers 1973). This was the first document to list the S-175 optimum headwater 

stage at 4.50 feet. The original design settings at S-174 were lowered by 0.20 feet 

following a request by the Central and Southern Florida Flood Control District in March 

1968. This request was made following an examination of ground surface elevations in 

the area above S-174 which indicated higher stages could not be tolerated during the 

agricultural growing season. All subsequent operational changes were taken from the 

structure operations logs provided by the South Florida Water Management District. S174 and S- 175 were constructed to pass the design discharge with a 0.50-foot head loss 

across the structures. For this reason, all stages listed in Table 2 are for the headwater 

recorder only. 

From July 1971 to June 1982. S-174 was set to maintain an upstream stage of up to 

5.50 feet in the L31N canal and to open full when the headwater stage exceeded 5.80 

feet. Two minor short term operational changes (lowering of the open setting from 5.80 

to 5.70 at S- 174) occurred during July 1979 and from November 198 1 to May 1982. In 

June 1982 the operational settings at structure S-174 were reset to maintain a stage of 

up to 4.60 feet upstream of the structure in the L31N canal and open full when 

headwater stage exceeded 4.90 feet. This change allowed lower water levels to be 

maintained in the upstream reaches of the L31N canal providing additional flood 

protection for the residential and agricultural areas adjacent to the canal. The change 

also greatly increased flow into the L31W canal. One additional minor change In the 

operations settings occurred in June 1983 when the settings for S-174 were raised by 

0.10 feet. 
Table 2. Summary of Initial Design Settings and Operational Changes at 

Structures 5-174 and S-175. 

INITIAt DESIGN SETTINGS 

(L3 1W Detail Design Memorandum, 1967 and 

cofrespondence dated March 1968) 

S- 174 SPILLWAY S- 175 CULVERTS 

5.50 ft. open 5.00 open 

5.30 ft. static 

5.10 ft. closed 

FIRST OPERATIONS SETTINGS* 

(SFWMD Operations logs, July 197 1, prior to 

this date structures were closed or on) 

S- 174 SPILLWAY S-175 CULVERTS 

5.80 ft. open 4.50 open 

5.50 ft. static 

5.10 ft. closed 

NEW OPERATIONS SETTINGS 

(SFWMD Operations logs, June 1982) 

S- 174 SPILLWAY S- 175 CULVERTS 

4.90 ft. open 4.50 open 

4.60 ft. static 

4.40 ft. closed 

NEW OPERATIONS SETTINGS 

(SFWMD Operations logs. June 1983) 

S- 174 SPILLWAY S- 175 CULVERTS 

5.00 ft. open 4.50 open 

4-70 ft. static 

4.50 'ft. closed 

NEW OPERATIONS SETTINGS 

(Shark Slough Delivery Test Agreement, July 1985) 

S- 174 SPILLWAY S- 1 75 CULVERTS 

4.50 ft. open 4.50 open 

4.30 ft. static 

4.10 ft. closed 

The onb definite settings for S-175 that could be found are those from 

the iniiial design settings listed above and the settings listed in the 

South Dade Conveyance System General Design Memorandum (U.S. 

Army Corps of Engineers 1973). For all other dates they were assumed 

to have remained unchanged. 
Operations criteria for the control structures in the L31W canal have been 

controversial since the late 1970's. Farmers have requested that stages be lowered in 

the Frog Pond area late in the wet season to allow earlier land preparation and planting. 

Everglades National Park argued that this would be detrimental to park water resources 

since it causes drainage of northern Taylor Slough. In 1981. representatives from the 

Water Management District, the National Park Service, and the Frog Pond farmers met 

to develop recommendations for Frog Pond area water management. The group agreed 

that wet season optimum water levels of 4.50 feet should be maintained at the 

upstream side of structures S-175 and S- 177 for as long as local rainfall remained 

adequate. It was further agreed that the South Dade Conveyance System facilities 

should be utilized to maintain 3.00-foot headwater stages at these structures in up to a 

one in 10 year drought as long as sufficient water was available. It was expected at that 

time that planting in the Frog Pond area would begin when water levels receded 

naturally below a headwater stage of 4.00 feet at structure S-177. During the growing 

season, structures S-175, S-177, and S-332 would be operated to alleviate any 

agricultural flooding problems that might occur as a result of large rainfall events 

(approximately five inches or more). 

Between August and November 1984. the operational settings at structure S-174 were 

lowered by 0.20 feet as part of a wet season field test of water deliveries to Northeast 

Shark Slough. The operational settings were lowered by an additional 0.20 feet in July 

1985 in conjunction with a two-year experimental water delivery test in Northeast 

Shark Slough. These changes were part of an agreement between south Dade farming 

interests and the South Florida Water Management District. The operational changes 

allowed lower stages to be maintained in the upstream reaches of the L31N canal 

providing greater flood protection for the residential and agricultural areas during the 

tests. The change also increased flows into the L31W canal and prompted increased 

use of structure S-175. 

The Froe Pond Ex~erimental Drawdown Agreement 

In early 1984, Frog Pond area farmers stated that market conditions required that a 

schedule be developed allowing for earlier land preparation and planting. Negotiations 

with the National Park Service began. which lead to the Frog Pond experimental 

drawdown agreement. The agreement speciffed that water levels in the Frog Pond area 

would be lowered to allow preparation and planting to begin in mid-October as a onepear experiment evaluating the effects of the drawdown on the water resources of Taylor 

Slough (the full text of the agreement appears in Appendix A). Key terms of the 1984 

Frog Pond Agreement regarding structure operations were as follows: 

1. Water levels in the L-3 1W canal were to be operated such that on October 15. 

1984, the tailwater stage at structure S-174 and the headwater stage at structure S175 would be no higher than 3.50 feet NGVD. After the crops were planted. tailwater 

stages at S- 174 and headwater stages at S- 175 would be maintained at or below 3.50 

feet NGVD until the end of the growing season and harvest. 

2. Water levels in the C-111 canal were to be operated such that on October 15. 

1984. the tailwater stage at structure S-176 and the headwater stage at structure S177 would be no higher than 3.50 feet NGVD. After the crops were planted, tailwater 

stages at S- 176 and headwater stages at S- 177 would be -maintained at or below 3.70 

feet NGVD until the completion of the growing season and harvest. 
STUDY METHODS 

The Field Monitoring Network 

The Taylor Slough study area described in Figure 1 was divided into two subareas for 

most of the evaluation methods. The northern subarea extends from the park 

boundary west of structure S-174 southward to the main park road (Fig. 3). The 

southeln subarea extends from the main park road southward to approximately 2,500 

feet below the end of the L31W canal (Fig. 4). The initial hydrologic investigations, 

undertaken to evaluate the 1984-85 experimental canal drawdown, concentrated in the 

northern portion of Taylor Slough above the main park road. Only one water level 

station (R127) was examined south of this area. The results of this first-year study 

were summarized in an internal report (Wagner et al. 1985). The results of this study 

suggested that additional investigations were needed below the main park road to 

evaluate the effects of the large volumes of water released through structure S-175 

during the experiment. Prior to the 1985 drawdown period, new discontinuous 

monitoring stations were added to the study area south of the park road along with 

several new stations in key areas within the northern part of Taylor Slough. 

Tables 1 and 2 in Appendix B list the water level stations used for evaluation of the 

1984 through 1986 drawdown periods and identify the years of the study for which 

data were collected at each station. Three rainfall stations were also examined. These 

included continuous recording rain gages located at the S-332 pump station, 

maintained by the South Florida Water Management District. and stations R3110 and 

Royal Palm, maintained by the park (Figs. 3 and 4). 

Techniques for Documenting Drawdown Effects 

A number of approaches were used to determine the hydrologic effects of the 1984 

through 1986 drawdown periods including: 

1. Examination of continuous water level records to determine water level 

recession rates, response times, and maximum water level declines attributable to the 

canal drawdown operations. 

2. Development of water level contour maps to spatially describe the pattern of 

water levels in the northern Taylor Slough basin and allow comparisons of these 

patterns throughout the study periods. 

Drawdown Responses at Continuous Stage Recorders 

The first step in examining the effects of the canal drawdowns was to construct stage 

hydrographs for the continuous water level recorders in the L3 1W canal and in Taylor 

Slough. This was done by tabulating four-hour stage data for the 12 continuous stage 

reccrders within the study area. The length of this collection period varied each year in 

an attempt to include the period of time that was most affected by the drawdown 

operations. In 1984 this period was from October 12 to October 24. In 1985 the period 

was from October 8 to October 24 and in 1986 the period was from October 8 to 

October 26. The longer time periods for 1985 and 1986 were the result of significant L 

31W stage reductions occurring prior to October 15 in each of these years. 

A series of plots of these stage hydrographs have been provided in the results section of 

this report. There are two figures for each year of the study. The first is a plot of S- 175 

headwater stage versus a series of stations in Taylor Slough north of pump station 
Figure 3. Map of the northern subarea showing the locations of the monitoring 

stations for the canal drawdown study. 
Ffgrve 4, Map of the southern subarea showing the locations of the monitoring rtations for the canal drawdown study. 
S-332. The second is a plot of S-175 tailwater stage versus a series of st'ations 

downstream of -this structure. This has been done because the portion of Taylor Slough 

above the S-332 pump station showed the effect of the decreases in L31W stage while 

the portion of the basin below S-332 primarily showed the effect of increased discharges 

through structure S- 175 as a result of the drawdown operations. 

Determination of Background Recession Rates 

The effects of the canal drawdown operations were assessed by comparing the observed 

water level declines which occurred during the drawdowns with water level declines 

before and after the drawdowns. To do this, daily water level recession rates were 

determined for station R3110 in the northern portion of the basin during selected 

periods in 1984. 4985. and 1986. Station R3110 was selected because it is 

approximately 12,000 feet west of the L31W canal and was originally assumed to fall 

outside of the influence of the major canal drawdown effects. Daily water level 

recession rates were calculated for station I33110 during a three-month period from 

September 1 to November 30 for each of the three years. This three-month sample for 

each of the three years of the drawdown study covered a wide range of hydrologic 

conditions. A few general characteristics were common to all of the study years. For all 

three years of the study, the average daily water level recession rate was calculated at 

0.06 feet per day, and fell within a relatively narrow range of 0.03 to 0.09 feet per day. 

For each year the highest daily water level recessions (0.09 to 0.10 feet per day) 

occurred following large rainfall events. The lowest daily water level recessions in each 

year (0.01 to 0.03 feet per day) occurred following long periods of little or no rainfall, 

generally coupled with the absence of L31W structure operations. The calculated 

average background recession rates and other pertinent statistics for station R3110 

during the three study periods are listed in Table 3. The specific time periods used 

were selected based on the following criteria: 

1. The periods began at least two days after any recorded rainfall event and 

were within a period of receding water levels. (Water level recession rates tended tostabilize within 48 hours after a rainfall event.) 

2. The periods contained no discharges through structure S-175 and only 

minor discharges through structure S-174 (inflows less' than 40 cfs). (This provided 

relatively constant stages in the L31W canal and stable gradients between the canal, 

and the water level recorders in northern Taylor Slough.) 

3. Pumpages at structure S-332 during this period remained relatively constant 

and close to the required minimum delivery sc'hedule. (This made the downstream 

water level mounding effects of the pump station as constant as possible.) 

Periods fitting the above criteriaranged from two to 12 days in length, and began and 

ended at least five days from the-tober 15 drawdown date when water levels appeared 

to be stable. During 1984 and 1986, 23 days were used for the background recession 

rate estimates. In 1985 only 14 days fit the above criteria due to numerous rainfall 

events and extensive use of structures S-174 and S- 175. 

Development of Water Level Contour Maps 

Water level contour maps were developed using average daily stages for three dates 

bracketing the October 15 drawdown date during the 1984 and 1985 study periods. 

The 1986 study period was not contoured because the discontinuous field stations were 

not monitored and fnsumcient spatial data were available. The contour maps were 
constructed using the network of stage stations in northern Taylor Slough described in 

Figure 3. These maps are included in Appendix C. A listing of the stations used for 

the contour maps is provided in Table 1, Appendix B. 

Several computer contouring packages were investigated but they were abandoned 

because of their inability to accurately contour the near constant water level boundary 

in the L31W canal at the S-332 pump station. The final water level contour maps 

were created by interpolating between a minimum of the three nearest field monitoring 

stations. From this interpolation, one-quarter-foot contour intervals were plotted by 

hand. Additional water level control points were added in the L31W canal between the 

control structures to provide a more realistic estimate of the actual canal stages. The 

water levels at these points were linearly interpolated from the headwater and tailwater 

stages at each structure. This process forced the contour lines to intersect the canal at 

the control structures where the actual stage changes occurred, and allowed the canal 

to be treated as a near constant water level boundary. 

Table 3. Estimated Background Recession Rates in feet per day 

for Station R3110 during the Study Periods. 

1984 STUDY PERIOD 

PERIOD MEAN ST. DEV. MAX. MIN. N 

10/22-10/24 0.09 0.01 0.10 0.08 3 

10/29- 10/30 0.09 0.00 0.09 0.09 2 

1110-1121 0.04 0.02 0.07 0.01 12 

1 1 /25- 1 1 /30 0.06 0.02 0.08 0.04 6 

OVERALL 0.06 0.03 0.10 0.01 23 

1985 STUDY PERIOD 

PERIOD MEAN ST. DEV. MAX. MIN. N 

09/06-09/07 0.06 0.02 0.07 0.04 2 

1 1 /08- 11 / 14 0.05 0.02 0.08 0.02 7 

1 1 /26- 11 /30 0.08 0.0 1 0.09 0.07 5 

1986 STUDY PERIOD 

PERIOD MEAN ST. DEV. MAX. MIN. N 

09/24-09/29 0.07 0.01 0.09 0.06 6 

10/02-10/04 0.05 0.01 0.06 0.04 3 

10/07-10/ 10 0.06 0.01 0.06 0.05 4 

1 1 /03- 1 1 /05 0.06 0.0 1 0.07 0.05 3 

11/18-11/20 0.03 0.01 0.04 0.03 3 

1 1/24- 1 1 /27 0.04 0.02 0.07 0.03 4 

OVERALL 0.06 0.02 0.09 0.03 23 
RESULTS AND DISCUSSION 

HydroloPic Conditions and Structure Operations durinp 1984 

Daily rainfall, structure discharges, and average S-175 headwater stages for the period 

from September to December 1984 are summarized in Figure 5. This four-month 

period was selected because it covered the hydrologic conditions prior to the drawdown 

and included the most significant operational changes occurring in association with the 

Frog Pond drawdown agreement. Rainfall in the Taylor Slough basin during August 

and September 1984 was just slightly below the 37-year mean measured at Royal Palm 

Ranger Station (Table 1). During October. November, and December, rainfall dropped 

off rapidly and was well below average with no large rainfall events occurring after late 

September. During September and early October. prior to the agricultural drawdown, 

L31W canal stages generally remained above 4.00 feet (Fig. 5). 

From September 20 to October 11, structure S-174 remained open full to lower stages 

in the L31N canal system in association with a 90-day wet season test of water 

releases into Northeast Shark Slough. S-174 was closed at 0855 on October 12, and 

remained closed untfl January 21, 1985, except for minor releases on October 16 and 

the 17th. From September 20 to October 4, structure S-175 was used repeatedly to 

move excess water through the L31W canal system. S-175 was closed from October 4 

until 1120 on the 15th. when gates 2 and 3 were opened full for approximately three 

hours to achieve the required drawdown. By 0730 on October 16, gate 3 was closed, 

while gate 2 remained partially opened until October 2 1. S- 175 remained closed until 

March 2 1. 1985, except for minor releases between November 4 and the 9th. following 

several days of rainfall (Fig. 5). 

Structure S-175 remained open for seven days beginning on October 15, in order to 

stabilize water levels at the required 3.50 foot level. A total of 1.263 acre-feet passed 

through S-175 during this period with 589 acre-feet discharged during the first two 

days. During this same period. a total of 97 acre-feet passed through structure S-174. 

This indicates that a net total of 1,166 acre-feet was removed from the G31W canal via 

S-175 as a result of the 1984 drawdown. This water was presumably derived from 

several sources including storage in the L3 1W canal, seepage around S- 174, and from 

the aquifer underlying northern Taylor Slough and the Frog Pond agricultural area. 

Discharges from pump station S-332 were erratic throughout the month, but no 

attempt was made to mod@ pumpages during the drawdown experiment. 

1984 Drawdown Effects at Continuous Stage Recorders 

Stage hydrographs for the S-175 headwater recorder and three recorders west of the 

canal in northern Taylor Slough are summarized in Figure 6A for the period from 0000 

on October 12 to 2400 on the 23d. The hydrographs were developed using water level 

data taken every four hours from the recorder charts at each station. On October 11, 

the Water Management District conducted flow tests on structure S-174. At 0805, the 

structure was turned off and the gates were closed. At 1405. the structure was opened 

full and was not closed until 0855 on the 12th. From 0800 on October 13 to 0800 on 

the 15th. headwater stage at structure S-175 dropped 0.37 feet (from 4.18 to 3.81 feet) 

in response to closing S-174 on the 12th. This closure was in response to upstream 

canal stages dropping below 4.50 feet, and was not related to the canal drawdown 

agreement. Between 0800 on the 15th and 2400 on the 15th. headwater stage at S-175 
1984 RAINFALL AT S332 

S174 AND S175 DISCHARGE 

so07 

S175 HEADWATER STAGE 

Figure 6. 8-332 rainfall, S-174 and 8-175 structure discharges, and 8-175 

headwater stage during the 1984 drawdown period. 
OCTOBER 1984 

- T.S. BRIDGE -- 8158 *= Ria7 - S175T1 

OCTOBER 1984 

Figure 6 (A and B). Stage hydrographs for the continuous recorders upstream of 

S-175 (A) and downstream of S-176 (B) during the 1984 study period. 
dropped an additional 0.58 feet (from 3.81 to 3.23 feet) in response to opening S-175. 

In summary, the combined effects of the manual closing of S-174 on the 12th and the 

opening of S-175 on the 15th was a 0.95-foot decline in S-175 headwater stage (Fig. 

6N. 

In response to structure operations, large water level declines were observed at the 

continuous recorders in northern Taylor Slough close to the canal. These stage 

reductions diminished rapidly moving west. At station NTS 1, located 1,000 feet west of 

the canal, water level dropped 0.42 feet between 0800 on October 15 and 0800 on the 

16th. Station NTS3. located 3.300 feet west of the canal, showed a much lower stage 

reduction, dropping 0.25 feet during the same 24-hour period (Fig. 6A). At station 

R3110, located 12,000 feet west of the canal. the stage decreased 0.06 feet over the 24- 

hour period. Water levels at this station had been receding at a rate of 0.04 feet per day 

prior to the 15th. After the 24-hour period, the recession rate increased to 0.09 feet per 

day. 

The effects of increased dlscharges through structure S-175, resulting from the canal 

drawdown experiment, are summarized in Figure 6B for the tailwater recorder at S-175 

and several downstream stations, during the period from 0000 on October 12 to 2400 

on the 23d. The tailwater recorder at S-175 and the recorder at station R158 show 

clear responses to the increased discharges following the gate changes at S- 175 on the 

15th. The recorder at Taylor Slough bridge showed only a slight reduction in its 

recession rate wer this period. This station appeared to be controlled more by the 

operation of pump station S-332 than by the operational changes at S-175. Station 

R127, at the southern end of the study area, showed a slight decrease in its recession 

rate during the S- 175 drawdown period. 

Hydrologic Conditions an& Structure O~erations during 1986 

Daily rainfall, structure discharges, and average S- 175 headwater stages for the period 

from September to December 1985 are summarized in Figure 7. Rainfall in the Taylor 

Slough basin during August and September 1985 was below the period of record mean 

at Royal Palm and was significantly below. average throughout October (Table 1). 

During September and early October. L31W canal stages remained well above 4.00 

feet. Large rainfall events occurred in mid-October and again In mid-November, 

prompttng extensive use of structure S-175 to maintain G31W canal stages below the 

3.50-foot criteria established by the agreement between the fanners and the Water 

Management District (Fig. 7). 

On July 22 the operations settings for structure S-174 were lowered by 0.40 feet in 

association with the water delivery experiment in Northeast Shark Slough. At 1155 on 

October 1 1. S-174 was taken off automatic and opened full to lower G3 1N and G31W 

canals in preparation of the October 15 drawdown agreement. At 0845 on October 15, 

the gate was partially closed to begin the process of lowering L31W canal stages to the 

3.50 foot level. S-174 was not completely closed until November 19, except for several 

short periods in late October and early November. when it was closed temporarily to 

lower water levels following rainfall (Fig. 7). S-174 was operated manually throughout 

most of this period because of numerous rainfall events that required repeated gate 

changes to maintain the 3.50-foot downstream criteria. S- 174 remained closed through 

late February 1986 except for a minor release in early December. 

Structure S-175 was closed from October 3 until October 8 in order to maintain 

headwater stages as close to 4.50 feet as possible at the request of Everglades National 

Park. On October 8. gate 2 at S-175 was partially opened when the headwater stage 
1985 RAINFALL AT 5332 

S174 AND 5175 DISCHARGE 

-T 

S175 HEADWATER STAGE 

Figure 7. 5-332 rainfall, S-174 and 8-178 structure discharger, and 8-178 

headwater stage during the 1985 drawdown period. 

2 1 
exceeded 4.50 feet. At 1307 on October 10. gate 2 was opened full and by 1200 on the 

llth gate 1 was also opened full. At 0836 on October 12. gate 3 was opened full and 

the three gates remained fully opened until the 23d when gates 1 and 3 were closed. 

The operations logs for structure S-175 indicate that gate changes beginning on 

October 10 were for the purpose of lowering G31W canal stages to begin the drawdown 

to the 3.50-foot level required by the water delivery test agreement. On October 29. all 

three gates were again opened full, following rainfall events on the 16th and 17th. and 

remained open until November 6. S-175 was used again between November 18 and the 

25th and between December 6 and the 9th to maintain the 3.50-foot growing season 

criteria following rainfall events (Fig. 7). By October 16. S-175 headwater stage had 

stabilized at approximately 3.40 feet. For this reason, the 16th was used as the end of 

the canal drawdown period. Between October 10 and the 16th. S-174 discharged 3,024 

acre-feet into the L31W canal. During this same seven-day period, a total of 4,386 

acre-feet was discharged down the canal through structure S-175. This indicates that a 

net total of 1,362 acre-feet was removed from the G31W canal via S-175 as a result of 

the 1985 drawdown. Structure S-175 was used from the end of the drawdown period 

on October 16 until November 6 to maintain stages below the 3.50-foot level. This was 

prompted by numerous rainfall events during this period. S-175 was closed on 

November 6 but was opened full again on November 18 and the 20th due to several 

rainfall events and the threat from Hurricane Kate. Between October 16 and November 

6. S- 175 discharged 8,352 acre-feet down the G31W canal. During this time, S-174 

discharged 5.362 acre-feet into the canal. This indicates that a net total of 2,990 acrefeet were removed from the L3 1W canal via S- 175 as a result of this second drawdown. 

Although much of this water was due to local rainfall, these discharges were all made at 

canal stages below the established operational stage of 4.50 feet and would not have 

been required if the growing season drawdown agreement was not in effect. 

1985 Drawdown Effects at Continuous Stage Recorders 

Stage hydrographs for the S-175 headwater recorder and the four recorders west of the 

G31W canal in northern Taylor Slough are summarized in Figure 8A, for the period 

from 0000 on October 8 to 2400 on October 23. The large water level increases 

observed on the 19th were in response to a rainfall event in excess of one inch. From 

0000 on October 8 to 2400 on the 15th. headwater stage at structure S-175 was 

reduced from 4.60 feet to 3.38 feet, a decline of 1.22 feet. This reduction was 

accomplished by four separate gate changes at S-175 occurring on October 8. 10th. 

1 lth. and 12th. As stated earlier, the gate opening on October 8 was prompted by the 

S-175 headwater stage exceeding the 4.50-foot operating criteria established for the L 

31W canal and was not related to the water delivery test agreement. Between 2400 on 

October 10 and 2000 on October 1 1, S-175 headwater stage dropped 0.32 feet (from 

4.20 to 3.88 feet) in response to a change in the settings at gate 2 on October 10 and 

the opening of gate 1 on the llth. porn 2000 on October 11 to 2400 on the 15th. the 

headwater stage at S-175 dropped an additional 0.50 feet (from 3.88 to 3.38 feet) in 

response to the opening of gate 3 at S-175 on October 12 (Fig. 8A). In sumrfmy, the 

combined effects of the structure operations at S- 175 between October 10 and the 16th 

produced a decline of 0.82 feet at the S- 175 headwater recorder. 

Large water level declines were obsemd for the two park recorder stations closest to the 

G31W canal during the canal drawdown period. Smaller declines were observed at the 

stations further west of the canal. Station R3110 had mechanical problems and was 

not operational until 1200 on October 15. At station NTS 1, located 1.000 feet west of 

the canal, water level dropped 0.71 feet between 0000 on the 1 lth and 2400 on the 
a8 Iraiiiiiriiriiim 

881111l111llaa%aa 

01%s45878801as4 

OCTOBER 1985 

B 

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OCTOBER 1985 

Figare 8 (A and B). Stage hydrographs for the continuous recorders upstream of 

5-175 (A) and downstream of 8-175 (B) during the 1988 study period. 

23 
15th. At station NTS3. located 3.300 feet west of the canal. water level dropped 0.55 

feet during the same five-day period. At station NTS5, located 6.800 feet west of the 

canal, water level dropped 0.45 feet over the same period (Fig. 8A). 

Rainfall. between October 16 and the 19th. produced a 0.40-foot rise in the headwater 

stage at structure S-175 (Fig. 8A). This prompted a second period of canal drawdowns 

to maintain the 3.50-foot canal stage required .by water delivery test agreement. From 

0800 on October 20 to 0800 on the 23d. S-175 headwater stage dropped 0.60 feet (from 

3.72 to 3.12 feet) in response to a reduction in the gate setting at structure S-174 on 

the 20th and the closure of S-174 on the 21st. The S- 174 operations log indicates that 

these changes were made to maintain the required growing season criteria. At station 

NTS1, water level dropped 0.66 feet over the same three-day period. At stations NTS3 

and NTS5, water levels dropped 0.62 and 0.42 feet, respectively, over the same period 

(Fig. 8A). At station R3110 water level dropped 0.18 feet during this period which is 

equal to the calculated background recession rate for this station. 

Stage hydrographs for the S- 175 tailwater recorder and several downstream stations are 

summarized in Figure 8B, during the period from 0000 on October 8 to 2400 on the 

23d. The tailwater recorder below S- 175 and the recorder at station R158 showed clear 

responses to the increased discharges following the gate changes at S- 175 beginning on 

the 10th. The Taylor Slough bridge recorder did not appear to respond to the increased 

discharges at S-175. This station did show an increased water level recession rate 

between the 10th and the 13th that appeared to be related to decreased discharges 

through the S-332 pump station. R127 again showed only a slight reduction in its 

natural recession rate during this period. 

Hydrologic Conditions and Structure O~erations during 1986 

Daily rainfall. structure discharges, and average S-175 headwater stages for the period 

from September to December 1986 are summarized in Figure 9. FWnfall in the Taylor 

Slough basin during September, October. and November 1986 was well below the 

period of record mean at Royal Palm (Table 1). This trend continued until several large 

rainfall events occurred between December 24 and January 4. During September and 

early October, t31W canal stages were generally below 4.00 feet (Fig. 9). 

Structure S- 174 was turned off and opened full from September 3 to the 19th. following 

several large rainfall events and high stages in the upstream reaches of L31N. S-174 

was closed on September 19 and remained closed until May 1987, except for a nine-day 

period beginning on November 12 when required deliveries were made to Taylor Slough. 

Structure S- 175 was opened full from August 11 to September 2. when all three gates 

were closed. Gate 2 was partially opened again on September 5, when headwater 

stages approached 4.50 feet. By September 8, all three gates were opened full. S- 175 

remained open full until September 19. when all three gates were closed coinciding with 

the closure of S-174 (Fig. 9). By October 8. S-175 headwater stage dropped below 3.50 

feet and continued to decline until the 15th. Between 1600 on October 15 and 0800 on 

the 16th. headwater stage at S-175 rose 0.89 feet (from 2.96 to 3.85 feet) following a 

1.50-inch rainfall event on the 15th. At station R3 1 10, water level rose 1.55 feet (from 

3.18 to 4.73 feet) in response to this rain event (Flg. 9). At 1400 on October 16, gate 2 

at S-175 was partially opened for approximately 24 hours to lower canal stages down to 

the required 3.50 feet. During this period, S- 174 remained closed and S- 175 

discharged a total of 173 acre-feet down the canal. 
S174 AND S175 DISCHARGE 

SOOT 

S 175 HEADWATER STAGE 

Figure 9. S-332 rainfall. S-174 and 5-175 structure discharges. and 8-175 

headwater stage during the 1986 drawdown period. 
1986 Drawdown Effects at Continuous Stage Recorders 

Stage hydrographs for the S- 175 headwater recorder and the four recorders west of the 

canal are summarlzed in Figure 10A. for the period from 0000 on October 8 to 2400 on 

the 25th. From 0800 on October 16 to 2400 on the 17th. S-175 headwater stage 

dropped 0.55 feet (from 3.85 to 3.30 feet). At stations NTS1. NTS3. and NTSS. water 

levels dropped 0.39. 0.28, and 0.11 feet, respectively, over the same period (Fig. ION. 

At station R3110, water level dropped 0.16 feet which was slightly greater than the 

average background recession rate calculated for the station. 

Stage hydrographs for the tailwater recorder at S-175 and the three downstream 

recorders are summarized in Figure 10B, for the period from 0000 October 8 to 2400 

October 25. Large water level increases were obsemd at the three recorders north of 

the park road in response to a rainfall went on the 15th. Only minor discharges were 

made through S-175 following the rainfall event and their impact could not be isolated. 

The recorder at station R127, located at the southern end of the study area. received 

much less rainfall and showed only a slight decrease in the nod recession rate. 

The water lwel declines observed at the headwater recorder at structure S-175 and the 

four park water level recorders west of the canal in northern Taylor Slough are 

summarfzed in Table 4, for the 1984 through 1986 drawdown periods. The periods 

selected in each year correspond to the peak drawdown periods observed in Figures 6, 

8. and 10 in the previous sections. For each period, the actual observed water level 

reductions for each station are listed along with the calculated average background 

recession for station R3110. The 1984 canal drawdown occurred during a period of 

below normal rainfall. For this reason, the 0.06 feet per day average background 

recession is slightly greater than the observed water level decline at station R3 1 10 prior 

to the canal drawdown. The canal drawdowns during 1985 and 1986 occurred during 

periods of moderate to high rainfall when the background recession rate would have 

been slightly greater than average. The data set used for the background recession rate 

estimation was too small to estimate the rate under all conditions; as a result. Table 4 

contains the calculated background recession for all periods based on a recession rate 

of 0.06 feet per day. 

During the 1984 drawdown period, S-175 headwater stage declined 0.58 feet between 

0800 on October 15 and 0800 on the 16th. following the opening of S- 175. The average 

background recession for this period was 0.06 feet. producing a net decline of 0.52 feet 

as a result of canal drawdown operationd changes. At NTSl and NTS3 west of the 

canal. net water level declines of 0.36 and 0.19 feet, respectively, were estimated for the 

24-hour period, after the 0.06 foot .background recession was subtracted. At station 

~~3110, water level declined 0.06 feet which was equal to the calculated average 

background recession rate. 

DurIng the 1985 drawdown period, the headwater stage at S-1.75 was reduced 0.82 feet 

between 0000 on October 11 and 2400 on the 15th. This followed three gate changes at 

S- 175. Net water level declines at stations S- 175, NTS1, NTS3, and NTS5 of 0.52, 0.41. 

0.25, and 0.15 feet. respectively, were calculated for the five-day period. after the 0.30 

feet background recession was subtracted. Station R3110 was not functional during 

this period. Rainfall was under 0.10 inches which supported the appropriateness of the 

average background recession rate. Large water level declines were again obsenred 

between 0800 on October 20 and 0800 on the 23d. following a large rainfall event on 

the 19th. Net water level declines at stations S-175. NTS1, NTS3, and NTS5 of 0.42. 
8.8 8 

8811111111118828838 

018S4587880183458 

OCTOBER 1986 

*- R127 - S175TW 

OCTOBER 1986 

Figure 10 (A and B). Stage hydrographs for the continuous recorders upstream of 

S-175 (A) and downstream of 5-175 (B) during the 1986 study period. 
'Table 4. Summary of Observed Water Level Declines during the 1984 

through 1986 Drawdown Periods. 

1984 DRAWDOWN PERIOD 

START OF ENDOF S-175 l'V'IS1 NTS3 NTSS FC3110BACK. 

PERIOD PERIOD STAGE STAGE STAGE STAGE STAGE REC. 

0800 10/15 0800 10/16 0.58 0.42 0.25 ---- 0.06 0.06 

1985 DRAWDOWN PERIOD 

STAWT OF END OF S-175 NTS1 NTS3 NTS5 R3110 BACK. 

PERIOD PERIOD STAGE STAGE STAGE STAGE STAGE REC. 

oo00 lO/ll 2400 10/15 0.82 0.71 0.55 0.45 ---- 0.30 

0800 10/20 0800 10/23 0.60 0.66 0.62 0.42 0.18 0.18 

1986 DRAWDOWN PERIOD 

START OF END OF S-175 NTSl NTS3 NTS5 FC3110 BACK. 

PERIOD PERIOD STAGE STAGE STAGE STAGE STAGE REC. 

0800 10/16 2400 10/17 0.55 0.39 0.28 0.11 0.16 0.10 

All water level declines are in feet. Back Rec. represents the background 

recession calculated for the period based on a water &el recession rate of 0.06 

feet per day. 

0.48, 0.44, and 0.24 feet, respectively, were observed for the above three-day period, 

after subtracting the 0.18 feet background recession. Station R3110 had a water level 

decline of 0.18 feet during this period, which agrees exactly with the calculated 

background rate. 

During 1986 water levels in the L31W canal were well below the 3.50 foot drawdown 

criteria prior to October 15. Large water level declines were observed between 0800 on 

the 16th and 2400 on the 17th following 1.49 inches of rainfall on the 15th. Most of 

the water level declines can be attributed to the increased recession rates produced by 

the rainfall event. Net water level declines at stations S-175, NTS1, and NTS3 of 0.39, 

0.23. and 0.12 feet. respectively, were estimated for the 40-hour period. These net 

declines were calculated by subtracting the 0.16-foot observed recession rate at station 

R3110 during this period. It should be noted that the recession rate in the canal was 

more than double that of station R3110. These increased recession rates at the 

stations in and adjacent to the canal are presumably in response to the opening of S175. 

Water Level ContourinP Results 

A few general statements can be made about the spatial water level patterns in the 

northern Taylor Slough basin durhg the study periods. All of the water level contour 

maps (Figs. 1 through 6 in Appendix C) indicate that the regional groundwater flow 

direction in the northern Taylor Slough basin was toward the southeast. This general 

trend was disturbed by the canal drawdown operations which produced a strong west 

to east hydraulic gradient, with the L31W cd acting as a groundwater slnk. This 

change in the regional flow direction was reflected in the shape of the water level 

contour lines in the areas adjacent to the canal. 

The S-332 pump station produced a downstream mound in the water level patterns 

whenever it was in operation. The difference in head across the pump station created a 

large hydraulic gradient between the canal and the wetlands adjacent to the structure. 
This promoted flow of a portion of the S-332 discharges back into the canal making the 

pump station less effective as a water supply structure. Downstream of the pump 

station, flow appeared to diverge with surface water flows heading southwest through 

the bridge and culverts along the main park road while the groundwater flow continued 

toward the southeast. A large head difference was maintained across structure S- 175 

whenever the culverts were closed. This produced another large hydraulic gradient 

toward the canal in the sections immediately downstream of the structure. 

1984 Contour Results 

The contour map for October 9, 1984, shows the regional groundwater'flow pattern in 

the study area prior to the October 15 canal drawdown (Fig. 1, Appendix C). Structure 

S- 175 had remained closed for the previous five days while S- 174 was manually opened 

full to lower stages upstream in the G31N canal. L31W stage was much lower than 

the adjacent wetlands wen with the continuous inflows through S-174. This caused 

the canal to drain the adjacent areas producing the west to east gradient in the 

groundwater contour lines north of the S-332 pump station. On the 9th. this canal 

drainage effect appears to extend west of the canal to a distance of approximately 7,000 

feet. West of this point, groundwater flow maintained the southeastern regional flow 

direction. The S-332 pump station had a relatively localized effect on the general water 

level pattern on this date. Downstream of S-175 another large west to east gradient 

was produced by the head difference across the closed structure. 

By October 15. the canal drainage effect extended west of the canal to a distance of 

approximately 8,000 feet (Fig. 2, Appendix C). In the area north of the S-332 pump 

station, water levels declined by approximately 0.50 feet between the two contour dates. 

These declines were slightly greater then the estimated background recession rate for 

the period. The accelerated declines appeared to be in response to the closing of S-174 

on the 12th and the opening of S-175 on the morning of the 15th. As the water levels 

fell throughout the study area, the downstream mounding effect of the S-332 pump 

station became much more apparent. By the 15th. this mounding effect extended 

westward approximately 3,500 feet. At the same time, the gradient downstream of S175 was reduced following the opening of the structure on the morning of the 15th. 

By October 23. the canal drainage effect had stabilized, wlth the area of influence 

remaining approximately 8.000 feet west of the canal (Fig. 3, Appendlx C). Between the 

15th and the 23d, water levels had dropped an additional 0.50 feet in the area north of 

the S-332 pump station; but the overall pattern had changed only slightly from that of 

the 15th. The downstream mounding effect of the S-332 pump station continued to 

expand. By this date, the water level mound extended approximately 4,500 feet west of 

the canal. Downstream of S-175, the gradient increased again following a reduction in 

discharges on the 17th and the closure of S-175 on the 21st. 

1985 Contour Results 

The contour map for October 7, 1985, shows the regional groundwater flow pattern in 

the area prior to the canal drawdown (Fig. 4, Appendix C). Structure S- 175 was closed 

on October 3d and remained closed until the 8th. S- 174 had remained open full since 

early September to maintain upstream stages below 4.50 feet, as part of the Northeast 

Shark Slough experimental agreement. The contour map indicates that October 1985 

water lwels were much higher in the area adjacent to the L31W canal than in 1984. 

The higher water levels were the result of continuous inflows into the canal via S-174. 

and rainfall between the 4th and the 7th. This produced a relatively flat water table in 

the area north of the S-332 pump station. Under these higher stage conditions. the 
mounding effect of the pump station remained localized. High rainfall and the closure 

of S- 175 on October 3 produced a gradient of. 1.72 feet across structure S- 175. This 

created a large west to east gradient downstream of the structure. 

The October 15 contour map shows the effect of the lowering of G31W canal stages 

beginning on the 8th (Fig. 5. Appendix C). As stated earlier, this initial opening was in 

response to S-175 headwater stage exceeding 4.50 feet and is not related to the 

drawdown agreement. The structure was opened further on the 10th and by the 12th. 

S-175 was opened full. Both of these operational changes were made to begin the canal 

drawdown process. The headwater stage at S-175 declined 1.23 feet between the 7th 

and the 15th. The October 15 contour map indicates that the canal drainage effect 

extended west of the canal to a distance of appnurimately 7.000 feet. As water levels fell 

throughout the study area, the mounding effect of the S-332 pump station became 

much more apparent. By the 15th. this mounding effect extended westward approximately 4,500 feet. The gradient downstream of S-175 was reduced as a result of the 

continuous use of S-175 which equalized upstream and downstream stages. 

By the 21st, the canal drainage effect appeared to have stabilized and the area of 

influence was reduced to approximately 6,000 feet (Fig. 6, Appendix C). Between the 

15th and the 21st. the L31W canal and adjacent stage recorders had declined less 

than 0.10 feet. The western stage recorders had a slight increase in water levels which 

produced a flattening out of the water table in this area. The contour map for the 2 1st 

also indicates that the downstream mounding effect of S-332 was reduced. This was 

the result of a discontinuation of pumping on the 20th. The gradient downstream of S175 was further reduced following 14 days of continuous operation of S- 175. 

 he 1984 through 1986 Canal Drawdown ODerations in Relation to Omrations 

During Prfor Periods 

Prior to the 1984 drawdown agreement, planting in the Frog Pond agricultural area 

would normally begin &er stages in the C- 1 ll canal had naturally receded below 

approximately 4.00 feet. During this period, there was no formal agreement to use the 

G31W and C-111 canals to provide additional flood protection below the established 

operational settings summarized in Table 2. The daily water level records for structure 

S-175 were examined to determine the appraximate dates at which the t31W canal 

stages had receded to 3.50 feet prior to the 1984 agreement. Table 5 shows the dates 

that the L31W canal stage (S-175 headwater) receded to 3.50 feet after October 1 of 

each year during the period from 1970 to 1983. The table indicates that recessions to 

3.50 feet prior to October 15 occurred only Once during the 14-year period. The average 

date of recessions to that level was November 20, five weeks after the October 15 date 

specified by the kog Pond and Northeast Shark Slough experimental agreements. 

SUMMARY AND CONCLUSIONS 

Examination of the historic boundaries of Taylor Slough (Fig. 1) indicates that the 

mafority of the Frog Pond agrlcultural area and a large portion of the East Everglades lie 

within the headwaters of the basin. For this reason. management of water levels in the 

Frog Pond area and the lower G31N canal system is crucial to the water resources of 

the Taylor Slough basin in Everglades National Park. Examination of stage 

hydrographs and water level contour maps generated in this study indicate a strong 

Interaction between the drawdown operations in the L31W canal and water levels in 
Table S. Dates.at which L-31 Stages Receded to 3.50 

feet after October 1, 1970, to 1985. 

YEAR DATE 

1970 November 12 

1971* October 4 

1972 November 3 

1973 November 8 

1974 October 2 1 

1975 November 12 

1976 November 25 

1977 October 3 1 

1978 November 27 

1979 November 2 1 

1980 December 7 

1981 December 9 

1982 January 17 

1983 December 20 

The mean date for recession to a stage of 3.50 feet was 

November 20. G31W canal stage was considered to be 

the headwater stage at S- 175. * S-175 was unavailable, 

so S-174 taflwater was substituted. 

northern Taylor Slough. Water level declines were observed throughout the northern 

portion of the Taylor Slough basin during the 1984 through 1986 canal drawdown 

operations. The 1984 canal drawdown was undertaken as a one-year experiment 

through an agreement between the National Park Service and a group of farmers in the 

Frog Pond area. The canal drawdowns during 1985 and 1986 were made in association 

with a two-year experimental delivery test in Northeast Shark Slough. This 

experimental test was established by an agreement between the South Florida Water 

Management District and a larger group of south Dade farmers, and authorized by 

Public Laws 98- 18 1 and 99- 190 (U. S. Congress 1983, 1985). 

During the 1984 drawdown period. water level declines were observed in response to 

two major operational changes. Between October 13 and the morning of the 15th. S175 headwater stage fell 0.37 feet in response to the closing of structure S-174. This 

closure was related to routine upstream water management practices and was not 

associated with the drawdown agreement. The 1984 drawdown period began on the 

morning of October 15. By the morning of the 16th. S- 175 headwater stage had 

dropped 0.58 feet. At park stations NTS1. NTS3, and I33110 west of the canal. water 

levels dropped 0.42, 0.25, and 0.06 feet, respectively. over the same period. Structure 

S-175 remained open for seven days to stabilize G31W canal stages at the required 

3.50 feet. During this period. the drawdown operations removed a total of 1,166 acrefeet from the G31W canal via S- 175. Water level recession rates at station R3 110 

showed a two-fold increase during the week of the drawdown. This indicates that the 

drawdown impact extended west of the canal at least 12.000 feet into Everglades 

National Park. 
The largest water level declines of the three study years occurred during 1985. Wall 

and upstream water management operations associated with the Northeast Shark 

Slough experimental agreement prompted nearly continuous use of structures S-174 

and S-175 throughout the 1985 study period. Reduction of G31W canal stage to 3.50 

feet as required by the Northeast Shark Slough experimental agreement was 

accomplished through three separate operational changes. Operations logs for 

structure S-175 indicate that the 1985 canal drawdown began on October 10. Between 

October 11 and the 15th. S-175 headwater stage dropped 0.82 feet. At park stations 

NTS1. NTS3, and NTS5, water levels dropped 0.71, 0.55 and 0.45 feet. respectively. 

during the same period. By October 16, S-175 headwater stage had stabilized at 3.40 

feet. Between the 11th and the 16th. the canal drawdown operations had removed a 

total of 1.362 acre-feet from the L31W canal via S-175. Rainfall between October 16 

and 19. 1985, prompted a second canal drawdown. S-175 had remained open full 

throughout this period so the canal stage lowering was accomplished by reducing S- 174 

inflows. From October 20 to the 23d, S-175 headwater stage dropped 0.60 feet. At 

park stations NTS 1, NTS3, NTS5, and R3110, water levels dropped 0.66, 0.62, 0.42, 

and 0.18 feet. respectively, over the same period. Structure S- 175 continued to be used 

until November 6. Between October 16 and November 6, canal drawdown operations 

removed an additional 2,990 acre-feet from the L3 1W canal via S-175. 

During the 1986 study period, headwater stage at S-175 had dropped below 3.50 feet 

prior to the October 15 date established in the Northeast Shark Slough experimental 

agreement. By the morning of October 15. S-175 headwater stage had declined to 2.98 

feet following a long period of below normal rainfall. Rainfall during the evening of the 

15th produced a 0.87-foot rise in stage at the S-175 headwater recorder. S-175 was 

opened on the 16th for a 24-hour period to lower canal stages to the required 3.50 feet. 

During this period. S-175 discharged a total of 173 acre-feet down the L3 1W canal. 

From the 16th to the 17th. S-175 headwater stage dropped 0.55 feet. At park stations 

NTS~, ~~$33, NTS5, and R3 110, water levels dropped 0.39, 0.28, 0.11, and 0.16 feet, 

respectively, over the same period. The relatively small discharge through S- 175 during 

this period suggests that most of these declines were a natural response to the higher 

recession rates produced by the rainfall event. The two-fold increase in recession rates 

at the stations in and adjacent to the L31W canal was presumably in response to the 

opening of S-175 on the 16th. 

Water level recessions in the L31W canatbelow 3.50 feet by October 15 were almost 

unprecedented prior to the 1984 experimental agreement occuning only once during 

the previous 14 years. The average date of water level recessions to 3.50 feet under 

previous water management operations was November 20, five weeks later than the 

date of the 1984 experimental drawdown. Durlng the period from 1980 through 1983, 

L31W canal stages never receded belaw.3.50 feet prior to December 7. Prior to the 

1984 drawdown experiment. there yas no formal agreement to use the L31W canal to 

provide additional flood protection below the established 4.50-foot operational setting. 

The hydrologic investigations conducted during the 1984 through 1986 study periods 

indicate that the L31W canal agricultural drawdown operations produced significant 

water level reductions throughout northern Taylor Slough, particularly i.n the areas 

adjacent to the L31W canal. All of these drawdowns occurred during years when the 

total annual rainfall was over 10 inches below normal and monthly total rainfall during 

the September through November study periods was also below nonnal (Table 1). This 

suggests that the hydrologic impacts of future agricultural canal drawdowns could be 

much greater. 
The L31W borrow canal was designed "to convey flow to replenish the fresh water 

supply in the Taylor Slough area of the park" (U.S. Army Corps of Engineers 1967). 

Structure S-175 was originally designed to "maintain water levels up to 5.00 feet and 

provide a means of either diverting borrow canal discharge overland into Taylor Slough 

or passing it south to be distributed overland from the borrow canal south of State 

Road 27" into central Taylor Slough (U.S. Army Corps of Engineers 1963). Following 

this initial design study, S-175 headwater stage was reduced by 0.50 feet and set to 

maintain upstream optimum water levels of up to 4.50 feet for as long as local rainfall 

remained adequate (U.S. Army Corps of Engineers 1973). 

The overall results of this study indicate that management of wet season stages below 

4.50 feet in the upper portion of the L31W canal causes drainage of the adjacent Taylor 

Slough wetlands. Review of water level data collected during the last four and one-half 

years of the Frog Pond drawdown study indicate that dumg the months of June 

through November, L3 1W canal stages generally remained well below the stage in the 

Taylor Slough wetlands. Under these conditions, the L31W canal acts as a 

groundwater sink draining the adjacent wetlands. This is inconsistent with the initial 

design purpose of the G31W canal which states that the canal should replenish the 

fresh water supply to the Taylor Slough basin. The lowering of L31W canal stages to 

allow earlier land preparation and planting in the Frog Pond agricultural area further 

exacerbates the drainage effects of the canal system and is incompatible with the 

system design intent of restoring fresh water flows to the Taylor Slough basin. 
LITERATURE CITED 

Earle. J.E., and J.H. Hartwell. 1973. Hydrologic Data for Taylor Slough. U. S. 

Geological Survey. Open File Report No. 73012. Tallahassee. Florida. 

Jarosewich, M., and J.I. Wagner. 1985. Geologic Structure of the Surflcial Aquifer 

System ilnderlying Everglades National Park and the Big Cypress National Preserve, 

South Florida Research Center Report 85-02. Homestead. Florida. 

Rintz. RE.. and L.L. Loope. 1978. Vegetation Map of Taylor Slough, Everglades 

National Park. South Florida Research Center. Homestead. Florida. 

Rose, P.W.. M.D. Flora, and P.C. Rosendahl. 1981. Hydrologic Impacts of L31W on 

Taylor Slough, Everglades National Park. South Florida Research Center Report T6 12. Homestead. Florida. 

Schorner, N.S.. and RD. Drew. 1982. An Ecological Characterization of the Lower 

Gverglades, Florida Bay, and the Florida Keys, U. S. Fish and Wildlife SeIvice. 

Washington D. C. 

U. S. Army Corps of Engineers. 1961. Survey Review Report, Central and Southern 

Florida Project, South Dade County, Jacksonville Florida. 

U. S. Army Corps of Engineers. 1963. Central and Southern Florida Project, Part V., 

Supplement 37, General Design Memorandum. Southern Dade County. Jacksonville, 

Florida. 

U. S. Army Corps of Engineers. 1967. Central and Southern Florida Project. Part V.. 

Supplement 47. Detafl Design Memorandum, Levee 31 (west). Canal 113, and 

Control Structures S- 174 and S- 175. Jacksonville. Florida. 

U. S. Army Corps of Engineers. 1973. Central and Southern Florida Project, Part V.. 

Supplement 52, General Design Memorandum. Conveyance Canals to Everglades 

National Park and South Dade County. Jacksonville. Florida. 

U. S. Congress. 1948a. Comprehensive Report Plan. Central and Southern Florida 

Project. House Document 643,80th Congress, Washington D. C. 

U. S. Congress. 194813. Public Law 80-858.80th Congress, Washington D. C. 

U. S. Congress. 1954. Public Law 83-780,83d Congress. Washington D. C. 

U. S. Congress. 1962. Public Law 87-874.87th Congress. Washington D. C. 

U. S. Congress. 1968. Public Law 90-483.90t.h Congress, Washington D. C. 

U. S. Congress. 1970. Public Law 91-282,91st Congress, Washington D. C. 

U. S. Congress. 1983. Public Law 98- 18 1, 98th Congress, Washington D. C. 

U. S. Congress. 1985. Public Law 99-190.99th Congress. Washington D. C. 
U. S. Department of Agriculture. 1958. Soil Survey (Detailed-Reconnaissance) of Dade 

County, Florida, U. S. Department of Agriculture, Series 1947, No. 4, Washington D. 

C. 

Wagner. J.I., D.J. Grigsby, and RA. Johnson. 1985. Effects of the 1984 "Frog Pond 

Drawdown upon the Northern Taylor Slough Water Resources, South Florida 

Research Center, Report to the Superintendent. Homestead. Florida. 
Appendix A. The 1984 "Frog Pond1' Drawdown Agreement. 

AGREEMENT fiE: "FROG PONDm DRAWDOWN 

THIS AGREEMENT entertd into this 4th day of 

June 

*y, 1984, by and between the, Individuals and Corporations 

farming in the 'Froq Pondm area (hereinif ter collectively 

called "Farmersm) and the Everglades National Park will 

apply to the 1984-1985 planting season, and is not to be 

construed as having any precedential or binding effect 

whatsoever on the Farmers or Everglades National Park in 

other. future years or operations. 

The parties hereto being advised that the South 

Florida Water Management District will .honor any agreement 

entered into by them concerning mdrawdowns' in the area of 

Dade County, Florida known as the "Frog Pond,. it is 

thereupon agreed that for the 1984-1985 planting season, the 

parties agree that the following water levels are 

acceptable : 

1. Water levels in L-31W shall be operated in 

such a manner so that on October 15, 1984 the headwater 

level at Structure 5-175 and the tailwater level at S-174 

will be no higher than 3.5 msl. Thereafter, after the 

Farmers have planted thelr crops, headwater levels ct S-175 

and tailwater levels at S-174 will not be allowed to exceed 

3.5 msl until the crops are harvested. 

2. Water levels in Canal C-111 will be operated 

in such a manner so that on October 15, 1984 the headwater 

level at Structure S-177 and the tailwater level at S-176 

will be no highei than 3.5 msl. Thereafter, after the 

Farmers have planted their crops, headwater levels at S-177 

and tailwater levels at S-176 shall not be allowed to exceed 

3.7 msl until the crops are har.vested. 

3. The Everglades National Park will monitor a 

series of wells in the Park west of L-31 West. The South 

Florida Water Wariageaent District will monitor discharges 

and upstream.'downstream stage at structures 5-174, 5-175, 
Appendix A, continued. 

5-176, S-177 and S-332, an4 will establish and monitor l 

water level recorder in the Frog Pond. All water level and 

discharge data gathered from these wells 2nd structures will 

be made available to all the undersigned. 

4. By this Agreement, the Farmers &a not concede 

that ef that Everglades National Park or tbe South Florida 

Water Management District have any 'authority to do anything 

in the Vrog Pondm area to diminish the flood protection 

given the area under the Central & Southern Flood Control 

DATED this 4th 

EVERGLADES NATIONAL PARK 

// Super intendent, 

Everglades National 

Park 

June 

day of Ma+! - 1984. 

-- /-. 

for Individuals and Corzmra- --. -- tions farming in the 'Frog 

Pondm area. 

STATE OF FLORIDA 

) SS: 

COUNTY -OF DADE ) 

BEFO'OKE RE, the undersigned authority, personally 

appeared JOHN M. MOREHEAD, known to me to be the person 

referred to in the foregoing Agreement and who acknouleciqed 

before me that he had authority to execute this Agreement 

and to enter into this Agreement On behalf of Everglades 

National Park. 

ny Commission expires: 

m1c STATE Cf.ROIIM AT UIQ 

Y.\' C~I~SlON wlm MAT 22 1984 . WJND TmU GiMW IW . UlllMllU 

STATE OF FLORIDA 

) 3s: 

COUNTY OF DADE 1 

I &cCC/zE, I. 

.NOT#RY PUBLIC 

appeared WILLIAM G. EARLE, ESQ., 

individuals and "frog Pond" 

area. 

My Commission expires: . . . . . . . '.Z.T.. 1". .. . :1:: #,.; F.,.:..;: 

i': . ,.,.a,.: :,., :%.;!... :-; d. .e .--: 
Appendix B. 

Table 1. Stations in the Northern Subarea used in the 1984 through 

1986 G31W canal drawdown study. 

STATION TYPE OF SAMP. YEARS 

NAME DATA FREQ. N STUIIIED 

S- 174TW 

L31WN 

NTS8 

NTS9 

NTS 10 

NTSll 

R3 1 10 

NTSl 

NTS2 

NTS3 

NTS4 

NTS5 

NTSl2 

Ell1 

S-332HW 

S-332'IW 

NTS6 

NTS7 

El 12 

El 10 

S- 175HW 

S- 175'IW 

El 13 

El 14 

El 15 

NTS13 

L31W 

R158 

E 158s 

El57 

El56 

E155N 

E155S 

El54 

E 153N 

E153S 

TSB 

E 1 52N 

E 152s 

NTS14 

CANAL STAGE 

CANAL STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

CANAL STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

CANAL STAGE 

CANAL STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

CANAL STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

corn. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

CONT. 

CONT. 

DISC. 

CONT. 

DISC. 

CONT. 

DISC. 

DISC. 

corn. 

CONT. 

DISC. 

DISC. 

DISC. 

DISC. 

corn. 

CONT. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

CONT. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

COW. 

DISC. 

.DISC. 

DISC. 

Indicates that the station was used In the 1984 or 1985 water 

level contour analysis. 
Appendix B, continued 

Table 2. Stations in the Southern Subarea used in the 1984 through 

1986 G3lW canal drawdown study. 

STATION TYPE OF SAMP. YEARS 

NAME DATA FREQ. N STUDIED 

WETLAM) STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

CANAL STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

WETLAND STAGE 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

DISC. 

CONT. 

SAMP. FREQ. -- Refers to type of water level measuring equipment 

used at the station. 

CONT. = Continuous water level recorder. 

DISC. = Discontinuous measurements taken from a st& gage or 

a well. observed every six to 32 days. 

N -- Refers to the number of tfrnes during the 1984 and 1985 field 

collection period that water levels were collected at this station. 
Appendix C. Water Level Contour Maps (1984-85) 

9 OCT. 1984 

Figure 1. Water Level Contour Map for October 9, 1984. Average Daily Water 

Levels were 4.10HWJ2.83TW at 5-175 and 4.lbHW/5.31TW at S-332. 
Appendix C, continued 

16 OCT. 1884 

-- 

Figure 2. Water Level Contour Map for October 16, 1984. Average Dafly Water 

Levels were 3.54HW/2.87TW at &I76 and 3.69HW/S.27TW at 6-332. 

4 1 
Appendix C, continued 

Figure 3. Water Level Contour Map for October 23, 1984. Average Daily Water 

Levels were 3.32HW/2.46TW at S-175 and 3.3SHW/5.13TW at S-332. 

42 
Appendix C, continued 

7 OCT. 1986 

Z 

r 

8-176 

F'igure 4. Water Level Contour Map for October 7, 1985. Average Daily Water 

Levels were 4.68HW/2.93TW at 8-175-and 4.73HW/5.02TW at 8-332. 
Appendix C, continued 

Figure 5. Water Level Contour Map for October IS, 1985. Average Daily Water 

Levels were 3.42HW/3.31TW at 5-175, and 3.51HW/S.24lW at S-332. 
Appendix C, continued 

21 OCT. 1 @86 

Figure 6. Water Level Contour Map for October 21, 1985. Average Daily Water 

~evels were 3.37HW/3.31TW at S-175 and 3.45HW/4.43TW at 8-332. 

45