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Posted 9/6/2013

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By Erica Skolte


A diagram of Lake Okeechobee, with arrows that show water flowing into the lake from the north and flowing out of the lake to the east, west and south may look simple; however, the reality is much more complex.

Many think that water naturally flows south. Though that is sometimes true, there are exceptions to the rule. For example, the St. Johns River flows north, and then turns east toward the Atlantic Ocean. The Caloosahatchee River flows west, and the south fork of the St. Lucie River flows north. In natural systems, water flows “downhill,” from areas of higher elevation to areas of lower elevation.

Lake Okeechobee is part of the highly-managed Central and Southern Florida system that makes it possible for people to live in south Florida. Around Lake Okeechobee, on any given day, the direction that the water flows depends on what is happening in south Florida.

Lake Okeechobee is the second largest freshwater lake completely within the continental United States, second only to Lake Michigan. It is approximately 37 miles long and 30 miles wide and covers 730 square miles. At an elevation of 15 feet, it holds four million acre-feet of water. It is surprisingly shallow for its size, with an average depth of only nine feet. Historically, it was larger in area than it is today, and during high water stages, it flowed freely over the south rim of the lake, into the Everglades. Today, the depth varies, depending upon the weather and the time of year, and the water must be managed to accommodate both. Lake levels are managed according to the Lake Okeechobee Regulation Schedule, and to maintain the integrity of Herbert Hoover Dike (HHD), constructed around the lake in the 1930s.

It is often said that HHD “surrounds” Lake Okeechobee, but it is not a completely closed, water-tight system. The current configuration of HHD was completed in the 1960s, with 143 miles of 35-foot high embankment around the entire lake, except a small area around Fisheating Creek where there is an open area with long tie-back levees on the north and south sides of the creek. Another reason that the dike is not “watertight” is that some water normally seeps through and under the dike, in the same way that groundwater moves naturally underground.

Additionally, there are 32 federal culverts and 28 other structures along the dike, including nine pump stations, ten spillways and nine navigation locks. These structures are operated by or the responsibility of the U.S. Army Corps of Engineers or the South Florida Water Management District. The operation of these structures, including the direction and volume of the flow of water, changes depending upon the closely monitored conditions in the lake and south Florida.

Where does the water that comes into Lake Okeechobee originate? Ultimately, the entire freshwater system is driven by rain, or the lack thereof. Rain may fall directly on Lake Okeechobee, or collect in a variety of basins that flow into Lake Okeechobee.

Water generally flows into Lake Okeechobee from several sources, including the Kissimmee River, Fisheating Creek, Lake Istokpoga, Taylor Creek and smaller sources such as Nubbin Slough and Nicodemus Slough. The Kissimmee River is the largest source, providing more than 60 percent of the water flowing into Lake Okeechobee. Fisheating Creek is the second largest source for the lake, with about nine percent of the total inflow.

The “headwaters,” or main source of water, in addition to rain, for Lake Okeechobee and the Everglades, is the 3,000-square mile Kissimmee Basin north of the lake. It begins near Orlando, Florida, with tributaries such as Reedy Creek, Shingle Creek and Boggy Creek, and includes more than two dozen lakes in the Kissimmee Chain of Lakes, their tributary streams and associated marshes and the Kissimmee River and floodplain.

Many of the lakes in the chain of lakes are connected by canals that are easy to trace using the satellite view in programs such as Google maps. Beginning at East Lake Tohopekaliga (known as East Lake “Toho”), move south through several canals and lakes into the Kissimmee River, and finally, Lake Okeechobee. Along the way are sections of Kissimmee River that were channelized (the C-38 canal is straight) and remnant oxbows of the historic river, as well as the 43-mile long Kissimmee River Restoration area, where 40 square miles of historic river floodplain has been successfully restored.

Historically, the Kissimmee River meandered for 134 miles and the floodplain was up to three miles wide. Bounded by Live Oaks, this vast floodplain was filled annually by seasonal rains. Today, the Kissimmee Basin is highly managed, but it continues to contribute a significant amount of flow into Lake Okeechobee.

Additional water collects in the 850-square mile Fisheating Creek watershed before flowing into the western part of Lake Okeechobee. It includes Fisheating Creek, the C-41 and C-41A canals, and Indian Prairie between Lakes Istokpoga and Okeechobee.

Significant amounts of rainwater can collect over the 5,600 square mile watershed that drains into the lake from the Kissimmee and other large basins. Three inches of rain over the watershed can result in a one-foot rise in the elevation of Lake Okeechobee. Since many of the water bodies are connected, the water does not stay in one place. It moves from lake to lake via the canals, carrying along whatever it encounters, including urban, agricultural and industrial runoff.

Agriculture is the major land use in the Kissimmee River and Fisheating Creek watersheds, occupying about 45 percent and 70 percent of each basin, respectively. Citrus production occurs throughout both basins, and cattle ranching and dairy farming are also extremely important. Cattle and dairy farms are significant sources of nitrogen, and fertilizers may be the source of both phosphorus and nitrogen in agricultural runoff.

During a weather event, water coming into the lake from direct rainfall, large basins and other sources could result in water levels that rise six times faster than water can be discharged from the lake. Just one foot of rainfall in the Kissimmee River Basin can cause the lake to rise as much as four feet within a few days. During hurricane season, June 1 through November 30, even a slow-moving tropical depression can drop a lot of rain over south Florida. Year-round, professionals predict weather trends and monitor lake levels to protect HHD as well as the rest of south Florida. During the dry season, water levels in the lake must also be monitored. The lake is part of the Okeechobee Waterway, and water levels must be maintained for navigation as well as the natural system, and water supply for urban areas and agricultural interests.

As for outflows, historically, Lake Okeechobee was not originally connected to either the east coast or the west coast. The first canal between Lake Okeechobee and the Caloosahatchee River was dug in 1883 by Hamilton Disston. The St. Lucie River was not connected to Lake Okeechobee until the St. Lucie Canal was constructed in the 1920s. Under the current system, if water must be released from Lake Okeechobee in order to reduce lake levels to protect HHD and the surrounding communities, the Caloosahatchee River and St. Lucie Canal serve as the primary outlets.

The 700,000-acre Everglades Agricultural Area (EAA) south of Lake Okeechobee was originally a wetland. Water spilled periodically over the banks of the lake, inundating the custard apple forest and sawgrass plains. Wetland plants decayed over time, forming the rich black soil that attracted sugar cane farmers and others to the area in the 1800s. Over-drainage, muck fires, oxidation and other factors have resulted in soil subsidence of more than six feet in some locations, making an already low-lying area into somewhat of a “sink.” The large canals that were dug to drain the area for agricultural use in the early 1900s must still be used to drain the area today. Water is moved out of the EAA and towards the coast in the West Palm Beach Canal, Hillsboro Canal, North New River Canal and Miami Canal. Since the EAA is fairly low in elevation, relatively little water is directly discharged into these canals from Lake Okeechobee. Under the current system, when south Florida is already saturated due to recent rains, discharges from the lake into the EAA would quickly raise the stages in the canals and groundwater levels in the towns and farms south of the lake.

As an emergency flood-control measure, water is sometimes discharged north into Lake Okeechobee, against the direction of flow that would occur naturally due to gravity. A Florida Department of Environmental Protection permit allows the pumping of water from the EAA into the lake when canal levels reach 12.5 feet. The South Florida Water Management District owns and operates three pumping stations at the north end of each of the large canals in the EAA, which can be used for these emergency operations:  the S-2 (at the north end of the intersection of the Hillsboro Canal and the North New River Canal), S-3 (at the north end of the Miami Canal) and S-4 (in Clewiston), which are built into HHD.



There are other constraints on the amount of water that can be moved south through the large canals and into the Everglades. Several stormwater treatment areas (STAs) naturally cleanse the water before it enters the Everglades ecosystem, which is naturally low in nutrients. These huge man-made filter marshes use submerged and emergent aquatic vegetation to pull nutrients such as phosphorus out of the water. However, the amount of water that can be moved south into the shallow STAs is limited, creating a bit of a bottleneck. If large amounts of water are pushed very quickly into the STAs, it “blows out” the treatment cells. It takes some time for the natural system to take up and hold the nutrients, and the process cannot be rushed.

There are other times when water flows in the opposite direction than expected. As lake levels have been high this summer, water has sometimes been moved north (not south) through the S-191 spillway structure on the north side of the lake, towards Nubbin Slough.

At culvert C-10A on the east side of the lake, water flow has been even more interesting this summer - water has flowed both into and out of the lake at various times. When residential areas in western Palm Beach County received heavy rain, water flowed out of the L-8 Canal and into the lake. Likewise, when the local basin is not as wet, water flows out of the lake and into the L-8 Canal.

 

The Corps manages the Okeechobee Waterway, on which a boat can cruise across the state of Florida. It is another part of the Central and South Florida system, and includes the St. Lucie Lock on the St. Lucie Canal in Stuart, the Port Mayaca Lock on the east side of Lake Okeechobee, the Moore Haven Lock on the west side of the lake, and the Ortona Lock near Moore Haven and the W.P. Franklin Lock in Alva, both on the Caloosahatchee River. With all of the locks and dams, water levels are different in each section along the waterway. Sometimes, when water is higher in the Caloosahatchee River or the St. Lucie Canal than it is in the lake, water may be allowed to backflow (upstream) into the lake. This may occur at Moore Haven during a low water event, such as when the lake level drops to 10 feet after an extended dry period. At Port Mayaca, when the canal is a certain level higher than the lake, water may be allowed to backflow into the lake to provide additional water for the lake or to take some water off of the local basin if necessary.

Water also moves into and out of the lake via groundwater and seepage. The floor of the lake is a porous limestone basin, full of holes, much like Swiss cheese, that is highly transmissive.

Another significant way that water leaves the lake is through evapotranspiration, a combination of evaporation and plant transpiration. The amount of evapotranspiration changes daily, but the big lake’s 730-square-mile surface area allows up to one-half inch a day of water to evaporate on a hot, cloudless day. During the dry season, and especially during periods of drought, evaporation alone can reduce lake levels significantly.

The south Florida water management system is complex, and water managers at the U.S. Army Corps of Engineers, Jacksonville District and its partners are committed to meeting its many varied yet interconnected needs.

Florida Herbert Hoover Dike HHD Jacksonville District Lake Okeechobee U.S. Army Corps of Engineers USACE