APPENDIX B - PHYSICAL CHARACTERISTICS
Definition of Management Boundaries
The Lake Tarpon watershed is 52.4 square
miles (KEA, 1992) with approximately half the watershed located in Pinellas
County and half in Hillsborough County (Figure B-1). A small portion of the watershed falls in Pasco County. The main tributary to the lake is Brooker
Creek. Pinellas County has identified
no less than fourteen channels that eventually converge to form the main
channel of Brooker Creek which discharges into Lake Tarpon on the southeastern
shore. The headwaters of Brooker Creek begin in the lakes region of northwest
Hillsborough County and account for the portion of the Lake Tarpon watershed
that extends into Hillsborough County.
Description of the Water Body and Its
Watershed
Lake Tarpon is the largest lake in Pinellas
County and has a surface area of approximately 4 square miles. The lake is approximately five miles long
and 0.75 to 1 mile wide. KEA (1992)
reported the average lake depth to be 7.3 feet with a maximum depth of 14
feet. The lake volume has been
estimated to be about 1 billion cubic feet (Bartos and Rochow 1976) with a mean
hydraulic retention time of 189 days (US EPA 1977).
Lake Tarpon, formerly called Lake Butler,
has historically been used for boating fishing and swimming. The lake was used for water supply for a four
year period between March 1926 and May 1930.
However, its use as a public water supply was abandoned due to the
frequent inflow of saline water through the Lake Tarpon Sink. Lake Tarpon is still widely used for
recreational activities.
The Lake Tarpon watershed has been divided
into three macro-basins referred to as the Lake Tarpon basin, the South Creek
basin and the Brooker Creek basin (KEA 1992).
The Lake Tarpon Basin which includes the lake proper and its immediate
shoreline, covers an area of approximately 9.1 square miles. The South Creek Basin, a 3.3 square mile
basin, encompasses an area draining to the Lake Tarpon Outfall Canal and Lake
St. George. Brooker Creek, the largest
of the basins, covers an area of approximately 42 square miles and extends into
Hillsborough County. Coastal (1995)
further subdivided the Brooker Creek basin into the gaged and ungaged Brooker
Creek basins (Figure B-2). The gaged
Brooker Creek basin is that portion upstream of the USGS gage located at
Ridgemoor Boulevard and the ungaged basin is that part located downstream of
the USGS gage.
Topographically the lake’s watershed ranges
from an elevation of less than five feet above mean sea level (msl) to greater
than 80 feet msl. The western and
northern most portions of the watershed are characterized by steeper slopes and
typically well drained soils, while the eastern area of the watershed is
generally flat and consists of poorly drained soils (KEA 1992). These soil and slope characteristics have
been an important factor in the development history of the watershed. Most of the commercial and high density
development in the 1950's and 1960's was generally concentrated in the western
portion of the watershed of the watershed.
This is important because this development pre-dated stormwater
treatment and wetland protection regulations.
During this time, much of the eastern shore of the lake and the Brooker
Creek watershed was in rural and agricultural land uses. Urban development in this area began in the
1970s and 1980s when more rigorous stormwater treatment and wetland protection
criteria were being adopted.
Of particular hydrologic significance is the
Lake Tarpon Sink located on the northwest shoreline. This sink, which is 118 feet in depth, was hydrologically
connected to Lake Tarpon. Dye studies
conducted in 1946 and 1949 confirmed a hydrologic connection between the Lake
Tarpon Sink and Spring Bayou in Tarpon Springs (Taylor 1953). The sink acted as both an outflow and inflow
depending upon the tide and the water level in the lake. Inflows from the sink resulted in increased
salinity concentrations in the lake. An earthen berm was constructed around the
sink in May 1969 by the District to prevent the exchange of water between the
sink and the lake. Until construction
of the Lake Tarpon Outfall Canal, the sink was the only surface water outflow
for the Lake. The Lake Tarpon Outfall
Canal was constructed as part of the Four River Basins Project by the US Army
Corps of Engineers to provide flood control for Lake Tarpon. The Outfall Canal located at the southern
most end of the lake was completed in 1967.
The Outfall Canal which is approximately 3.5 miles long and about 12 feet
deep connects the Lake to Upper Tampa
Bay. At the time of construction an
earthen dam was placed in the canal to prevent the backflow of salt water into
the lake. In 1971, the earthen dam was
replaced with an operable structure approximately 1.4 miles upstream of the
Outfall Canal’s confluence with Tampa Bay.
The Lake Tarpon Outfall Structure (S-551) is operated by the District
under the guidance of the US Army Corps of Engineers. The primary purpose of the Outfall Canal and S-551 is to provide
flood control for Lake Tarpon. However,
the Lake Tarpon Drainage Basin Management Plan (PBS&J 1998) has recommended
that S-551 be operated to provide water quality habitat benefits while
maintaining the District’s flood control objective.
Brooker Creek
Brooker Creek runs approximately 15 miles and drains approximately 42
square miles of northeast Pinellas County and northwest Hillsborough County
(IES 1993) before entering Lake Tarpon at its lower southeastern corner, less
than 3000 feet upstream of the Outfall Canal (SWFWMD 1989). The headwaters of Brooker Creek consist of
fourteen channels that eventually converge and form the main channel which
flows into Lake Tarpon. Five of these channels begin in the lakes region of
northwest Hillsborough County (Figure B-3).
Most of the channels of Brooker Creek are not well defined and can be
characterized as broad riverine wetlands (IES 1993).
Anthropogenic activities have impacted the
channels and wetlands of Brooker Creek.
The uppermost reaches of Brooker Creek in Hillsborough County have been
modified by ditches and water control structures that exist at Lake Keystone
and Island Ford Lake (IES 1993). In
addition to the typical impacts associated with urban development that exist in
both Hillsborough and Pinellas Counties, such as stream channelization for
flood control and filling of wetlands for development, there are there are
several potable water supply wellfields in northwest Hillsborough County. These impacts and low rainfall have resulted
in a decline in the average annual flow of the creek since 1961 (BWA
1978). Hydrogaphs for period of record
data from the District’s Resource Data Department are shown in Figures B-4 and
B-5.
Many of the channels of Brooker Creek pass
through the Brooker Creek Preserve in Pinellas County. The Brooker Creek Preserve consists of
approximately 8,000 acres of undeveloped land in the northeast corner of
Pinellas County adjacent to the Hillsborough/Pinellas County line. The land was acquired through a partnership
between Pinellas County and the District.
The Pinellas County Department of Environmental Management is
responsible for management of the County’s and the District’s lands within the
Preserve.
One of the most obvious environmental
impacts on the Brooker Creek Preserve is the existence of a large power line
corridor. This main corridor, as well
as a smaller power line and access roads, have bisected the channels of Brooker
Creek as they cross the Preserve. These
impacts include the filling of wetlands to construct the power line towers and
roads. In some cases, areas excavated
to provide fill have redirected flow away from the historic channels and
wetland areas.
The land around the mouth of Brooker Creek
is also owned by Pinellas County and managed as the John Chesnut, Sr.
Park. The 251 acre park offers a mix of
passive and active recreational activities.
Conservation areas make up about 103 acres and open space and recreation
areas about 148 acres.
Land Use
The Lake Tarpon watershed, inclusive of the
lake, encompasses approximately 52 square miles, with roughly half in Pinellas
County and half in Hillsborough County.
The Brooker Creek watershed accounts for 42 square miles of the Lake Tarpon
watershed and all of the Lake Tarpon watershed in Hillsborough County is in the
Brooker Creek Drainage.
In the Lake Tarpon Groundwater Nutrient
Study, ERM (1998) researched land use changes over the past 50 years and, as
can be expected, land uses within the entire Lake Tarpon watershed have changed
dramatically in the last 50 years. In
1950, upland forests and wetlands were the most dominant land use. Urban land uses, served by septic
tanks, were isolated to the Tarpon
Springs area, to small subdivisions along U.S. 19 on the southwestern edge of
the lake and around some of the lakes in northwest Hillsborough County. Citrus groves existed southwest of the lake
and pasture and range land were prevalent east of the lake. From 1950 to 1970 urban areas had expanded
somewhat. However, the most
prominent increase was in the amount of
land that had been planted in citrus.
This land use was widespread along the ridges southwest of the lake and
in the northwest Hillsborough area.
Between 1970 and 1990, urban growth replaced
much of the pasture, range land and citrus.
With the exception of the a small area near the east central shore of
the lake, the entire eastern margin of the lake had become urbanized and most
of these areas were served by small local package wastewater treatment
plants. Between 1990 and present,
Pinellas County upgraded their wastewater treatment system and these small
plants were hooked up to their regional wastewater treatment plant.
The most recent land use data available is
from 1995 and a map is shown as Figure B-6 and the acreage breakdown is shown
in Table B-1 below. From this
breakdown, it is evident that residential development dominates the land use
within the entire basin. This is followed
by wetlands, agriculture and open water.
Most of the wetlands and forest within Pinellas County occur east of the
lake and coincide with the Brooker Creek Preserve. Future development in Pinellas County likely will slow due to
the "built out" nature of the basin.
However, opportunities for additional development still exist especially
in Hillsborough County on the lands identified as agriculture.
Table B-1. Land use acreage within the Lake Tarpon
Study Area
|
Land use
|
Area (acres)
|
Percent cover
|
|
Agricultural
|
5966.4
|
16.2
|
|
Barren
|
.1
|
Less than 0.01
|
|
Commercial
|
2131.7
|
5.8
|
|
Forest
|
3764.7
|
10.2
|
|
Lake Tarpon
|
2472.4
|
6.8
|
|
Open
|
1089.47
|
3.0
|
|
Other Open Water
|
2472.8
|
6.8
|
|
Rangeland
|
709.5
|
1.9
|
|
Residential
|
9635.3
|
26.0
|
|
Trans, Comm, Utilities
|
404.8
|
1.1
|
|
Wetlands
|
8173.3
|
22.2
|
|
Total
|
36820.4
|
100.0
|
