Role of Flow-related Processes in Maintaining the Ridge and Slough
Landscape
Elizabeth Crisfield, Daniel Beard Research Center, Everglades National Park, Homestead, FL
Christopher McVoy, Everglades Division, South Florida Water Management District, West Palm Beach, FL
A quantitative picture of pre-drainage Ridge and Slough landscape features was developed from a synthesis of diverse historical sources including maps, photographs, quantitative U.S. General Land Office surveys, narrative descriptions from cross-Everglades expeditions, and historical aerial photos. Specific pre-drainage parameters identified include characteristic vegetation of ridges and sloughs; shapes, typical dimensions, and orientations of the landforms; and elevation differences between ridges and sloughs. In combination with narrative descriptions, these parameters provided a template representative of the pre-drainage landscape condition.
This template was then used to evaluate the current condition and types of changes found in the remaining areas of this landscape. The comparison focussed on three defining characteristics: (1) vegetation, (2) vertical dimensions (microtopography), and (3) spatial patterns. Areas known to have been altered by elevated P levels were excluded (NE Water Conservation Area 2A). Based on these characteristics and the assumed pre-drainage template, no presently remaining areas could be classified as “pristine.” The western portion of Water Conservation Area 3A most closely resembled the template, with a good match in spatial pattern. However, the degree of vertical relief appears to be less than one third of the original (“flattened microtopography”), and the sloughs have been invaded by emergent wet prairie species. Water Conservation Area 3B was the most severely altered, being affected in all three characteristics. Other areas were intermediate between these extremes. Overall, comparison with the historic template suggested extensive and significant degradation of the remaining Ridge and Slough landscape.
Significantly, an apparent endpoint for degradation of the Ridge and Slough landscape is conversion to a flat landscape in which the original elevation differences between ridges and sloughs completely disappear, and the landscape comes to form a uniform stand of sawgrass. Areas that appear to be at or close to this endpoint were found in northwest WCA 2A, northern WCA 3A, and WCA 3B.
In the second part of the investigation, we evaluated a number of recently proposed hypotheses concerning Ridge and Slough landscape maintenance for their ability to explain observed characteristics of the pre- and post-drainage systems. No single hypothesis appeared capable of explaining all the observed phenomena. However, a combination of water depths and water flows (and the post-drainage alterations in both) appeared to be most able to explain the full set of observations.
Because field research in this area is still in its infancy, we did not specify an exact flow-related mechanism in the above exercise; instead, we simply required that the mechanism be related to some aspect of water and material transport. As such the mechanism would have a directional component. Further research is required to distinguish between mechanisms related to transport of dissolved substances (e.g., organic carbon or nutrients), versus those related to transport of particulates (e.g., organic flocculent material). Research is also required to weigh the relative importance of average versus extreme transport events.
Full mechanistic understanding is likely to require a number of years of field and theoretical work. The degradation of substantial areas of Ridge and Slough landscape that has already occurred emphasizes the importance of restoration and prevention of further degradation. Ideally, restoration decisions would be based on a fully mechanistic understanding of this landscape’s geomorphology. Unfortunately, the ecological risks of further degradation will likely require decisions much sooner. Wherever possible, such decisions should make full use of the empirical information found in the landscape. So far, the landscape points toward the necessity of restoring both pre-drainage water depths and unimpeded, pre-drainage patterns of flow.
Elizabeth Crisfield, Daniel Beard Center, Everglades National Park, 40001 State Road 9336, Homestead, FL 33034. (305) 242-7822. .