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the role of flow in the everglades ridge and slough landscape >
The defining characteristics of the Everglades
With respect to vegetation, many of the plant species present in the Everglades are found elsewhere along the Gulf Coast, including the Mississippi River Delta (Penfound and Hathaway 1938). The delta of the Mississippi River also is characterized by moving water, and has plants common to the Everglades such as sawgrass (Cladium jamaicense) and maidencane (Panicum hemitomon). In addition, there are islands of bald cypress (Taxodium distichum) in the coastal marshes of the Mississippi Delta.
The role of moving water in wetlands
The movement of water in flowing water aquatic ecosystems is a fundamental construct of ecosystem structure and function, and its ecosystem role is well-established and studied. Flowing water has the ability to alter the landscape, which, in turn, has the ability to alter the landscapes ecology. Although much less studied in Everglades wetlands, the flow of water across vast expanses of low-gradient marshes was one of the defining characteristics of the river of grass. The flow of water in streams and rivers has been the topic of much research, and stream ecologists are well-versed in the importance of flow. Although water flow is much slower in the river of grass (typically less than one inch per second under present managed conditions [Ray Schaffranek, USGS, unpublished data]), it may be just as important to the ecosystem. Water movement plays a number of roles, including transporting, mixing, and diluting dissolved and particulate materials, the removal of metabolites, and influencing the location and type of aquatic organisms (Allan 1995). Many aquatic organisms orient their bodies to flow and use flow to direct their movements in the flow channel either positively or negatively (Welch 1963, Hynes 1970). The speed of the water flow also determines a number of physical features, including the size of eroded and suspended sediment particles, and the physical force that organisms experience in the water and on the bottom.
Research has been conducted on flow in other wetland systems that bear many similarities to the Everglades (Gregory Pasternack, personal communication). For example, the role of flow in floodplains, river deltas, and tidal marshes has been examined, and they share several characteristics with the Everglades, such as low gradients, dense herbaceous vegetation, fine-grained sediments, abundance of peat, and cohesive bottom substrates. These characteristics result in flow patterns that may be more complex than those of rivers and streams. Flow over a wide vegetated surface has dampened velocity fluctuations, higher turbulent dissipation, a complex vertical velocity profile, and significantly reduced lateral velocity profile relative to open channels.
Historical flow patterns
The central feature of the pre-drainage Everglades hydrology was a 30-mile-wide expanse of relatively shallow water moving downstream through the low-gradient wetland landscape. The pattern of water flow was remarkable for its regional uniformity across such a broad expanse, and for the absence of any central drainage channel or of any dendritic drainage pattern. Pine flatwoods formed most of the eastern boundary of this flow, and the western boundary was defined by the Immokalee Rise and the relatively higher wetlands and uplands of what is now the Big Cypress National Preserve. Much of the flow discharged south and west through Shark River Slough, through the mangrove estuaries of the southwestern coast, into the Gulf of Mexico (Fig. 1). South of, and including the New River (Ft. Lauderdale), the pine flatwoods were absent and the Atlantic Coastal Ridge became discontinuous, forming a series of islands separated by coastal rivers (Smith 1848). These rivers thus resulted in a portion of the flow being discharged eastward into Biscayne Bay and the Atlantic Ocean (MacGonigle 1896, Beard 1938). The remainder of the flow discharged southward through Taylor Slough into Florida Bay. Because of Floridas porous geology dominated by limestone overlain by thick peat deposits, the boundaries between surface water and ground water flow are not always distinct.
The Everglades ecosystem is thought to have been formed over the last 5,000 years as sea levels rose and precipitation increased, promoting water retention in a shallow inland basin, and the portion of the basin south of Lake Okeechobee filled in with peat (Gleason and Stone 1984). The result of peat accumulation in this bedrock basin was the formation of a peat surface, level in the east-west direction, and with a slight north-to-south downward slope. The concavity of the bedrock, coupled with the east-west levelness of the peat, resulted in thicker peat deposits in the middle of the basin and thinner deposits along the edges. By the 1880s, peat had accumulated to about 21 feet above sea level along the south shore of Lake Okeechobee (Meigs 1879), and had formed the northern edge of a north-to-south elevation gradient that is now less than 3 inches per mile. The southward flow of water down this gradient is thought to have formed and to maintain the ridge and slough pattern so characteristic of the Everglades (Kushlan 1993).
Everglades plant and animal communities evolved under the conditions imposed by this flow, within a sub-tropical climate, and under the constraints of nutrient limitation. The habitat types that evolved included vast sawgrass plains south of Lake Okeechobee, in the region presently occupied by the Everglades Agricultural Area. South and east of the sawgrass plains was the ridge and slough habitat, including what are now Water Conservation Areas 1, 2, and 3, and Everglades National Park. Marl prairies short hydroperiod marshes with marl sediments occupied slightly higher elevations east and west of Shark River Slough. The mangrove zone was located from the Ten Thousand Islands south and east around the tip of Floridas peninsula to the shores of Biscayne Bay.
Surprisingly, most discussions of hydrology in the Everglades exclude mention of the role of water movement. For example, Kushlan (1997) notes that hydropattern in wetlands typically is defined as the depth, duration, timing, and periodicity of water, but does not include water movement. However, he does note that the wetland water regime can influence the fate of water-borne sediment.
Historical ridge and slough landscape
Originally, the ridge and slough landscape consisted of a peat-based system of dense sawgrass ridges with soil surfaces roughly 2 to 3 feet higher than adjacent and relatively open sloughs (Wright 1912, Baldwin and Hawker 1915). The regular, approximately even spacing and the parallel arrangement of the ridges and sloughs, along with their alignment parallel with the direction of flow, formed a strongly organized pattern. Tree islands of various types formed a third element of the landscape, rising slightly above the elevation of the sawgrass ridges. The orientation and streamlined shape of the larger tree islands formed a separate pattern, but with the same alignment parallel to the direction of flow. The organized pattern of parallel ridges and sloughs, oriented with flow direction, on a slightly sloping peatland partially resembled other patterned, slightly sloped peatlands such as the circumpolar string bogs (Gore 1983). Historically, the ridge and slough landscape was an extensive pre-drainage landscape, encompassing what are now the Arthur R. Marshall Loxahatchee National Wildlife Refuge, Water Conservation Areas 2A and 2B,Water Conservation Areas 3A and 3B, and Shark River Slough (Fig. 1).
While seemingly small, the 2 to 3-foot difference in elevation between ridge surface and slough bottom was highly significant in the pre-drainage Everglades. During the typical annual rise and fall of wet and dry season water levels, this elevation difference allowed sloughs to remain water-filled throughout the year, while adjacent ridges would be exposed a few months of the year. In the pre-drainage system, native species were adapted to the multiple habitats provided by the tree islands, ridges, and sloughs. Aquatic organisms depended on the sloughs as extensive areas that would remain inundated throughout all but exceptionally dry years.
Drainage and compartmentalization activities
The first major efforts to drain the Everglades came with Hamilton Disston. By the 1890s, he had drained over 50,000 acres of wetlands, opened the Kissimmee River for navigation, and linked the Caloosahatchee River to Lake Okeechobee (Light and Dineen 1994). In addition, he is credited with excavating the first 11 miles of canal south of Lake Okeechobee in the direction of Miami the precursor to the Miami Canal. By 1917, four major muck-scalped canals transversed the Everglades from Lake Okeechobee to the Atlantic Ocean, short-circuiting the historic, north-to-south pattern of flow and greatly accelerating the removal of water from the Everglades. Unfortunately, flow records prior to disruption of the entire drainage basin are not available. It has been hypothesized that flow magnitudes through the river of grass were significantly higher in the 1800s and early 1900s versus what has been monitored since 1939 (Parker et al. 1955, Jon Woolverton, USGS, unpublished data). Patterns of peat subsidence (Stephens and Johnson 1951), water tables measured in 1915 (Baldwin and Hawker 1915), 1938 (Clayton 1938), and continuously between 1927 and 1939 (Parker et al. 1955), all suggest that these canals substantially lowered water levels in the northern Everglades, often below the peat surface.
Local efforts to surround Lake Okeechobee with a levee began in the early 1900s and were completed with the construction of a levee completely encircling the lake by the mid 1900s. All surface water inflow and outflow points (except one at Fisheating Creek) are now controlled by pumps and/or water control structures.
By the 1910s, Miami was expanding, and plans to drain and develop parts of the Everglades were pursued. Capt. Frank Jaudon, seeking to drain what is now called Northeast Shark Slough, successfully lobbied for creation of Tamiami Trail (Tamiami Trail Commissioners 1928). Objectors at the time (Tatum in Tamiami Trail Commissioners 1928) raised what turned out to be a legitimate concern, that the road bed would act as a dam, blocking the southward flow of water out of what is now Water Conservation Area 3B (c.f. Elliot in Graham 1951). Nevertheless, Tamiami Trail was completed in 1928. Photos from the 1930s show water occasionally spilling over the top (Matlack 1939), but aerial photos from 1940 (USDA-SCS 1940) suggest that in just 12 years, Tamiami Trail had created two separate landscape types, north and south, where once there had been a continuous landscape type.
Since that time, the ridge and slough landscape has been further compartmentalized. The eastern perimeter levee, stretching from Palm Beach to Miami-Dade county, defined the eastern boundary of the Everglades, and was completed by 1954 (Light and Dineen 1994). This levee became the eastern boundary of Water Conservation Areas 1 and 2, preventing movement of water eastward to agricultural and developing urban areas. By 1959, the Everglades Agricultural Area was separated from the rest of the system by a series of levees, canals, and water control structures. Subsequent levee construction defined the western and northern boundaries of the three Water Conservation Areas, and all three areas were completely surrounded by levees by 1963. A pair of parallel levees (L-67A and L-67C) divided Water Conservation Area 3 into two independent units. These levees, which run almost, but not quite perpendicular to the original sheet flow direction, were intended to reduce groundwater seepage through the highly porous Biscayne aquifer. The western unit, Water Conservation Area 3A, scheduled for higher water levels, serves as a major water supply reservoir. The eastern unit, Water Conservation Area 3B, with lower water levels, reduces the head difference to the developed areas to the east. In 1966, State Road 84 (now Alligator Alley), an east-west highway north of Tamiami Trail, was completed, adding another compartment to Water Conservation Area 3. Water conveyance was provided by bridges at one or more mile intervals.
Although not associated with levees, the so-called agricultural canals (e.g., Blue Shanty, Cooperstown) to the north and south of Tamiami Trail created further breaks in the landscape. The Miami Canal, also not associated with a levee, degrades the landscape not as a cross-barrier to flow, but as a shunt concentrating water flow in the canal and short circuiting the landscape.
Loss of ridge and slough landscape
It is clear that the compartmentalization and related water management activities, which have altered flow and other hydrological parameters such as water levels, are resulting in the loss of ridge and slough landscape. Loss is defined here in two ways: 1) a flattening of the landscape due to a decreased difference between ridge elevations and the elevations of the slough bottoms; and 2) an associated blurring of the distinct, directional pattern of ridge and slough vegetation. Evidence for this loss arises from alterations in vegetation patterns over time and indications of altered topography. Satellite images and aerial photography suggest subtle but definite changes in the extent and shape of sawgrass ridges. Where levees, canals, and roadways have separated the landscape into compartments, changes in the landscape are obvious. In areas where the original ridge and slough pattern still is evident, peat topography with identical patterning also is still evident. Where the vegetation pattern has been lost, the distinct topography also has disappeared. The post-drainage alterations in landscape topography show a clear trend from a highly organized, strongly directional pattern to a degraded, more random, and less directional pattern.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:04 PM(TJE)
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