the role of flow in the everglades ridge and slough landscape >
evidence of landscape change
Evidence of landscape change
Historical landscape condition
At the regional scale, the peat-based Everglades included two landscapes, the sawgrass plains at the northern end, and the larger ridge and slough landscape in the central and part of the southern portion (Fig. 1). The distinguishing differences between these two landscapes occur at the local scale, discussed in detail below. In the broader view, both of the landscapes were formed on a base of peat soil filling a bedrock basin. Although difficult to determine exactly, it appears that the pre-drainage elevation of the peat surface was about 3 feet below the elevation of the mineral soil-based edges of the basin, (e.g., the pine flatwoods east of the Everglades) (Van Zee et al. 1998). This elevation difference suggests that the basin peat accumulation probably was linked to average water depth.
The sawgrass plains and the ridge and slough landscape sloped very slightly from an elevation of about 21 feet above sea level at Lake Okeechobee (Meigs 1879) to sea level at Whitewater Bay 100 miles south, a slope of less than 3 inches per mile. Viewed from east to west, the peat-based landscapes were either level or, in some locations, tipped slightly to the east, reflecting drainage toward the Atlantic coast. Overall, at the regional scale and in the direction perpendicular to flow, both landscapes appear to have been very close to level over distances of more than 30 miles.
Sawgrass plains landscape
While the sawgrass plains and ridge and slough landscapes likely had similarly level surfaces at the regional scale, they differed markedly in topography at the local scale. The sawgrass plains formed a remarkably uniform, dense, and nearly monotypic stand of sawgrass, 8 to 12 feet tall. An 1883 expedition sponsored by the New Orleans Times Democrat did manage to penetrate and cross the sawgrass plains, but only by burning the sawgrass ahead of their progress.
The uniform sawgrass vegetation and the apparent absence of woody vegetation may reflect a very level peat surface.
Typical water depths in the sawgrass plains can be estimated from actual recorded water levels, from the nature of the soils and vegetation, and from studies of soil cores. The descriptions of a stringy peat with noticeable roots and sufficient structure to resist collapse (Stewart 1907, Harshberger 1914, Baldwin and Hawker 1915) suggest what would now be classified as a Fibric Histosol a partially decomposed peat (McCollum et al. 1976). The partial decomposition may have resulted from nutrient limitation in the oligotrophic Everglades, and/or from oxygen limitation because the soil was covered with water most of the year. Surface ponding for most, but not all, of the year also would be consistent with the optimal sawgrass growth conditions suggested by the great density and height of the sawgrass canopy.
Andrews observations are consistent with recorded pre-drainage water depths in areas of sawgrass growing on peat (i.e., the sawgrass plains and the peat transverse glades). Water depths in the 6 to 18-inch range, as well as the 24 to 36-inch range, were recorded in these areas. On the basis of pre-drainage soil and vegetation, and from the recorded historical water depths (Meigs 1879, Stewart 1907, Wintringham 1964), we estimate a typical annual variation in pre-drainage water depths between approximately 6 inches below ground surface at the end of the dry season, rising to a typical annual maximum of approximately 1.5 feet above ground at the end of the wet season. As historical descriptions suggest that sawgrass density and canopy height were similar on the ridges of the ridge and slough landscape, the water depth estimates from the sawgrass plains likely are applicable to sawgrass ridges of the ridge and slough landscape as well.
Ridge and slough landscape
In contrast to the uniform vegetation, topography, and hydrology of the uniform sawgrass plains, the ridge and slough landscape formed a distinctly non-uniform, systematically patterned peatland. The local and regional scale of these patterns were recognized clearly during early explorations of the Everglades.
Soil scientists who field-surveyed a 180-square-mile swath through the sawgrass plains and the ridge and slough landscape during the dry season of 1915 introduced the term ridges and sloughs to the scientific literature and found substantial differences in elevation between them.
Earlier, in 1907, members of a west to east survey across the Everglades also encountered alternating sawgrass ridges and open water sloughs which showed the same southeastward orientation later noted by Baldwin and Hawker (1915).
The photographs reproduced in Figs. 2 and 3 illustrate the ridge and slough landscape described above. They were taken in March, 1917, several months into the dry season. The photographer and explorer, John King, indicated that the Everglades already was drier than they had been prior to drainage. The open sloughs and distinct sawgrass ridges in these photographs are consistent with early descriptions of canoe travel. Col. Harney, and numerous others during the 19th century, found sufficient channels of open water to not only cross the Everglades by canoe, but to do so rapidly (Ives 1856, Dix and MacGonigle 1905, Dimock 1907, Marchman 1947, Church 1949, Anonymous 1960 ). These open sloughs allowed canoe travel, much of it using paddles, rather than the poles more commonly used in shallower waters. The Baldwin and Hawker survey conducted in 1915, a year of apparently normal rainfall, between January and March showed that even during the dry season, water levels apparently were high enough to cover both sloughs and ridges.
Having the benefit of aerial views and aerial photographs, hydrogeologist Garald Parker suggested that the directional orientation of the ridge and slough landscape was a natural result of water flow over a peat soil substrate.
The ridge and slough pattern is strikingly regular, with evenly spaced ridges (centerline to centerline), and only moderate variation in ridge width. Also striking is the absence of any indication of either a central drainage channel or of any dendritic, hierarchical drainage network. Instead, flow seems to have occurred quite evenly across the full 30-mile width of the Everglades, distributed quite equally across all the numerous sloughs. All of these observations support the regional impression of an unusually level peat surface in the cross-flow (east-west) direction.
The grain mentioned by Parker can be used to create a map of landscape directionality. Figure 5 shows the result of superimposing a 2-mile by 2-mile grid on the earliest available, comprehensive aerial photographs of the Everglades (USDA-SCS 1940), and then visually assigning a landscape direction to each grid cell. The uniformity of the landscape and flow directionality is striking, and reinforces the impression that peat accumulated in equilibrium with a regional water surface that was very level in the cross-flow (approximately east-west) direction. The mapping exercise also reinforces the impression of sheet flow distributed evenly across the full landscape width.
Figure 6 is a stylized depiction (exaggerated vertical scale) of the pre-drainage ridge and slough landscape illustrating the close relation between vegetation and elevation of the underlying peat substrate. Figure 6 helps illustrate the diversity of observations found in different narrative accounts of the pre-drainage Everglades. In particular, descriptions of the Everglades as a vast lake on the one hand can be reconciled with other descriptions of an alternating landscape of open water and boggy land on the other hand. At the end of the dry season, there could still be water in sloughs, yet the peat of the ridges could be slightly exposed, yielding boggy conditions on the ridges. In contrast, during the high water at the end of the wet season, water could cover both sloughs and ridges, yielding a continuous water surface a lake with emergent sawgrass stems.
Such a range of water depths is consistent with pre-drainage observed depths, soil characteristics, hydrologic preferences of the vegetation, and with hypothesized frequencies of peat fires. Several lines of evidence suggest that under natural, pre-drainage conditions, the water remained in sloughs throughout typical dry seasons. While there certainly is pre-drainage evidence of at least some sloughs drying out, such drying occurred infrequently. Also, there is evidence for year-round presence of surface water within sloughs during typical years, including ability to travel quickly by canoe (Smith 1848), lack of dry camping sites (Sprague 1848, Willoughby 1898, Dimock 1907), and absence of widespread peat fires (Gifford 1911, Cohen 1984).
There are many qualitative observations of water flow in the pre-drainage ridge and slough landscape. The majority of sources specifically noted the current of the water in the Everglades, making it clear that they did not observe stagnant areas and most also mentioned the direction of the flow.
Soil surveyors working in the spring of 1915 noted a visible current in sloughs, even in the dry season.
The botanist John Davis noted evidence of a current, even in the 1940s.
Current landscape condition
Post-drainage changes and vegetation patterns in the remaining ridge and slough landscape have been assessed using a variety of remote sensing data, including Landsat Thematic Mapper imagery, infrared aerial photographs, 1940s aerial photographs, and detailed vegetation maps. Float helicopter trips were used to examine the landscape at altitudes up to 1000 feet. GPS-linked photography was used to map particular observations within the overall landscape pattern. At this point, spatial analyses primarily are qualitative. As detailed vegetation mapping presently underway by the South Florida Water Management District is completed for Water Conservation Area 3A, it is expected that quantitative spatial analyses of vegetation polygon attributes will be helpful. The present lack of specific quantitative data at the landscape scale, however, does not negate the overwhelming degree of changes that are recognized easily by visual observation of aerial photographs, digital images, and maps.
With a template of the original ridge and slough landscape morphology in mind, it is possible to evaluate the condition of the currently remaining landscape. Figure 7 shows a present-day oblique aerial view of central Water Conservation Area 3A. Comparison with pre-drainage descriptions and with 1940s aerial photographs suggests that the original pattern has been well preserved in this area south of Alligator Alley and west of the Miami Canal. The exception to this pattern is a distinct band of altered vegetation immediately south of the Alley. Otherwise, this photograph is remarkable for its similarity with Fig. 4, the dirigible photo from the 1930s. Figure 7 and the area it represents provide our best estimate of an original ridge and slough landscape pattern. Figure 8, a vertical color infrared aerial, also is from central Water Conservation Area 3A. The directional pattern of entire, elongated ridges shown here serves as a good example of well-preserved ridge and slough landscape.
In contrast, Figs. 9-14 show oblique and plan view images and maps of degraded ridge and slough landscape. In each case, it is significant that comparison with the well-preserved pattern suggests a transformation from narrow, linear, and strongly directional polygons, to a post-drainage pattern of more randomly oriented, amorphous, and less directional polygons.
In Fig. 9, the landscape apparently has stayed wet enough to maintain open water and water lily sloughs, but the sawgrass ridges have disintegrated into many small, randomly oriented patches of sawgrass. In an aerial photo of the same region (larger area), faint traces of former sawgrass ridges are apparent (Fig. 10), as in much of the landscape north of Alligator Alley (I-75), and clearly show how much the landscape pattern has disintegrated in this area.
The degradation of the original ridge and slough pattern in this area and along the full length of Alligator Alley illustrates the impact of Alligator Alley construction and related water management practices. As can be seen in Figure 10, extra care was taken with this road to minimize the impacts of a hydrological barrier. Two canals were dredged adjacent to the road, one on each side to provide fill for the road. At every 2.5-mile interval (1-mile intervals, in some places), a small bridge underlies the roadway to connect the two canals and to attempt to make Alligator Alley hydrologically transparent within the Everglades landscape to the north and south.
However, despite the conveyance of water across the Alley at discrete intervals, the ridge and slough landscape has degraded substantially (Fig. 10). Negative landscape effects associated with the road have been even more severe farther west, between the Miami and L-28 Canals. The Alligator Alley experience is instructive, particularly given the efforts at maintaining hydrologic connectivity. The degraded ridge and slough landscape suggests that something more than hydrologic conveyance at discrete intervals is needed.
Figures 11 and 12 show a different form of ridge and slough landscape degradation, in which the sawgrass ridges appear to have remained intact. The sloughs, however, appear to be disintegrating, apparently by sawgrass invasion. The random orientation of the invading sawgrass contrasts with the original linear orientation of the ridges and sloughs. Although quantitative data have not been collected, it is possible that the sloughs are filling in with depositional sediments, as well as with vegetation. This process would result in severe degradation of the ridge and slough landscape, with the slough elevation eventually approaching that of the sawgrass ridges.
Figure 13 shows such an example of severe degradation of ridge and slough landscape in which the sloughs almost have been replaced completely by sawgrass. This degradation is typical for much of Water Conservation Area 3B. The pre-drainage expeditions that passed through Water Conservation Area 3B could not have traversed the landscape in its present condition. Figure 14 shows the same information as an excerpt from a vegetation map. A trace of the original directionality can be made out from the aligned remnants of sloughs. Even more complete replacement of ridge and slough landscape by uniform sawgrass has been observed in western Water Conservation Area 2A. Two peat transects (one-half mile each) measured in Water Conservation Area 3B show almost no discernible elevation differences (Chris McVoy, unpublished data). These transects support the relationship between flattening of the peat surface and loss of the characteristic ridge and slough vegetative pattern.
A 1980 vegetation map of part of Shark River Slough (Fig. 15) suggests that the process of ridge and slough landscape degradation and replacement by less directional, more amorphous landscapes also has occurred within Everglades National Park. The contrast in spatial pattern between the old and the new sawgrass is strikingly apparent.
Figure 16 is a satellite image of the western ridge and slough landscape which includes the locations represented in previous figures. This image provides the best available summary indication of the location of well-preserved versus degraded ridge and slough landscape pattern.
Flow in other regions of the greater Everglades
Surface flow obviously occurs in the freshwater and mainland estuarine wetlands of south Florida, and is not restricted to the Everglades ridge and slough system. For example, Tabb (1990) described flow in the broad marl prairies that border the southeastern flank of Shark River Slough. Tabb suggested that this flow occurred primarily during wet seasons when surface water levels in Shark River Slough were elevated enough to carry moving water onto and across the higher elevation prairies. Duever et al. (1979) showed possible flow patterns and directions during periods of high water for the entire Big Cypress National Preserve.
Flows from the Everglades to coastal estuaries are moderated by a coastal topography that has been shaped by water flow. The eastern fringe of the Everglades is bounded by a coastal ridge, interrupted by coastal rivers. On the lower southeast fringe, depressions in the ridge, called transverse glades, once carried fresh water to coastal wetlands, from where it was transported to Biscayne Bay by coastal creeks. Canals and levees lowered the water table, drained the transverse glades, and interrupted the flow of fresh water to coastal creeks. By analyzing aerial photography and sediment cores, Meeder et al. (2000) documented a substantial alteration in coastal creek topography that occurred sometime after 1940. The presence of small, buried oyster reefs near the mouths of these former creeks indicate that nearshore salinities at these sites were seldom greater than 20 ppt before 1940. Meeder et al. (2000) found that the creek bed area, once a topographic low that facilitated the transport of water, is now a topographic high that would retard water flow if water were available. Apparently, the flow of fresh water maintained these coastal creeks, acting as positive feedback that allowed future flow. When flow was redirected, a major landscape change took place. Observations by Glenn Simmons, reported in Simmons and Ogden (1998), suggest that a similar filling in of creeks that had previously facilitated freshwater flow to the coast across a low coastal berm (the Buttonwood Embankment described by Craighead ) may have occurred on the coast bounding northern Florida Bay.
Although it is beyond the scope of this paper to provide a detailed review of the role of flow in these other regions, it should be noted that much less attention has been given to flow in these landscapes than has been the case for the ridge and slough landscape.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:04 PM(TJE)