Impact of Water Management on Coastal Hydrology in Southeastern Florida
Robert A. Renken, Joann Dixon, A. C. Lietz, Pamela Telis, Alyssa Dausman,
Jeff Rogers, and Steven Memberg
U.S. Geological Survey, Miami, FL, USA
John Koehmstedt
U.S. Geological Survey, Reston, VA, USA
Scott Ishman
University of Southern Illinois, Carbondale, IL, USA
Richard Marella
U.S. Geological Survey, Tallahassee, FL, USA
Surface and ground water hydrology and natural ecosystems of southeastern Florida have been subjected to conflicting anthropogenic stresses, which are attributed
to the development of a highly controlled water-management system designed to
reclaim land for urban and agricultural development. During the first half of the
20th century, Everglades wetlands and coastal estuaries were viewed as a wasteland suitable only for drainage and development. Predevelopment surface- and
ground-water systems were intensely transformed by construction of a complex
system of canals, impoundments, control structures, levee systems, and many
large well fields. These features were collectively designed to manage the competing needs of agriculture, urban users, and natural ecosystem areas. Watermanagement systems were used to control floods, sustain ecosystems, prevent
overland flow from moving eastward and flooding urban and agricultural areas,
maintain water levels to prevent saltwater intrusion, and provide an adequate water supply.
Miami-Dade, Broward, and Palm Beach Counties have experienced explosive
population growth, increasing from less than 4,000 inhabitants in 1900 to more
than 3.8 million in 2000. The use of ground water, the principal source of municipal supply, has increased considerably, from 3 well fields producing 65 Mgal/d
(million gallons per day) in 1930 to more than 770 Mgal/d from 79 operating well
fields in 1995. Agricultural water use increased from 505 Mgal/d in 1953 to almost 1,150 Mgal/d in 1988, but has since declined to 764 Mgal/d in 1995. This
decline is partly due to the displacement of agriculture by urban growth. Presentday agricultural supplies are obtained largely from surface-water sources in Palm
Beach County and ground-water sources in Miami-Dade County, whereas agricultural growers in Broward County have been largely displaced.
Before 1948, surface-water conveyance canals provided unregulated flow and
were incapable of effectively transporting floodwaters. A lack of canal control
structures exacerbated overdrainage of the aquifer during periods of low rainfall
and drought. The Central and Southern Florida Flood Control project restructured
the existing conveyance system through canal expansion, construction of protective levees and control structures, and greater management of ground-water levels
within the surficial aquifer. Currently, gated canal control structures discharge
excess surface water during the wet season and remain closed during the dry season to induce recharge.
Managed surface-water conveyance has been used successfully to increase
ground-water levels near the coast to impede saltwater intrusion, minimize urban
80°45´
80°30´
80°15´
80°00´
and agricultural flooding,
27°00´
and maintain lower inland
PALM BEACH COUNTY
Lake
Okeechobee
ground-water levels. MeasWest
Palm
ured canal stage at the
Beach
26°45´
coastal reaches increased
during the latter half of the
EXPLANATION
20th century, whereas sur26°30´
face-water discharge to
coastal bays and the AtlanPALM BEACH COUNTY
tic Ocean declined. Stage
BROWARD COUNTY
26°15´
increase is presumably the
result of efforts to prevent
Fort
Lauderdale
saltwater intrusion into ma26°00´
BROWARD COUNTY
jor canals and aquifers. The
MIAMI-DADE COUNTY
decline in coastal surfacewater discharge is attributed
Miami
to municipal ground-water
25°45´
withdrawals that induce recharge from the canal to the
aquifer. The rerouting of
25°30´
surface water in major canals to secondary canals to
elevate coastal groundwater levels also may have
contributed to these deAverage difference in water levels between
clines. In contrast to coastal
October 1940-44 and October 1990-94
areas, long-term canal flow
near the western margin of the urban area has remained relatively consistent
HENDRY COUNTY
West Palm Beach Canal
North
N
COLLIER COUNTY
AVERAGE DIFFERENCES
IN WATER LEVELS
(FEET)
Can
al
-3.0
Atlantic Oce
an
Hills
boro
ew Riv
-2.0
-1.0
er Can
al
0
M
ia
mi
Ca
na
l
1.0
2.0
3.0
4.0
25°15´
0
e Bay
0
Bisca
yn
MONROE COUNTY
5.0
CANAL
(without a general increase or decrease in flow). Consistent long-term surface water flow along the western edge of urban areas and a decline in flow near the coast
further supports municipal well withdrawals as a causative agent of declining
coastal discharge.
Ground-water levels within the surficial aquifer respond quickly to rainfall; annual wet- to dry-season fluctuations within the aquifer can range from 2 to 8 ft.
Wide seasonal variability in water levels can tend to obfuscate longer-term patterns. To better illustrate the effect of canal drainage on the water table, long-term
“average condition” analyses were used to dampen storm and drought climatic
events. During 1990-94, wet-season (October) and dry-season (April) inland water levels within the surficial aquifer averaged 1 to 4 ft lower than the “average”
water levels experienced during 1940-44. However, coastal ground-water levels
averaged 1 to 2 ft higher, thus reflecting efforts to minimize coastal saltwater intrusion as previously described.
A broad zone of diffusion defines the saltwater interface, and its position is largely a function of lateral movement of seawater from the ocean, seepage from tidal
canals, and upconing of relict seawater. The predevelopment balance between
freshwater and saltwater was altered considerably following construction of conveyance and drainage canals and municipal supply wells. Saltwater intrusion has
been an issue of concern in southeastern Florida since the early 1930s; its effects
were most prominent in Miami-Dade and Broward Counties during the 1940s and
1950s, respectively. Canal drainage seems to have had the most widespread impact on saltwater intrusion, lowering water levels within the surficial aquifer and
contributing to landward movement of the interface
The pre-1900 Biscayne Bay coastal ecosystem and salinity was different than it is
today as recorded in sediment cores. Salinity increased in the early 1900s, remained stable until the early 1940s, and then increased above levels recorded in
the early 1900s. The marine fossil record shows that coastal vegetation changes
that have occurred. Sea grass became more abundant during the past century in
the central bay and in the coastal Manatee Bay region. Increasing epiphytal and
macro-algal habitat dwelling organisms indicate a change in substrate conditions.
From the late 1980’s to present time, salinity has decreased slightly in Manatee
Bay, and field observations in this region suggest the health of sea grasses is deteriorating.
Robert A. Renken, U.S. Geological Survey, 9100 N.W. 36th Street, Suite 107,
Miami, Florida, 33157, Phone: 305-717-5822, Fax 305-717-5801,
rarenken@usgs.gov, Hydrology and Hydrological Modeling