The Role of Flow in the Everglades Ridge and Slough Landscape
Science Coordination Team
South Florida Ecosystem Restoration Working Group
(Approved by the SCT: January 14, 2003)
The Everglades has at the core of its identity the slow movement of water across the vast, low gradient, wetland landscape. Marjory Stoneman Douglas eloquently immortalized this identity in her descriptions of the “River of Grass” (Douglas 1947). Drainage and compartmentalization efforts during the 20th century for flood control and water supply purposes interrupted this flow, as well as altering water levels, distribution, and seasonal timing. Water flows are closely linked to water levels, and their alterations have caused environmental damage. Efforts to restore the Everglades have focused on re-establishing more natural hydropatterns – the appropriate water levels, and the location, timing, and duration of these water levels. While these natural hydropatterns are widely recognized as being extremely important, much less attention has been paid to the importance of the actual movement of water, the physical and ecological roles that movement of this water plays, and how management activities have altered that flow. Thus, the Science Coordination Team has chosen understanding the science of the role of flow in the Everglades as one of its priorities. The purpose of this paper is to provide a stimulus for increasing the level of understanding and awareness of the role of flow in restoration activities, and to highlight the urgent need for research in this area.
Pre-drainage Everglades hydrology was dominated by a remarkable flow regime – a 30- milewide expanse of water moving down the low-gradient wetland landscape from north to south. Surprisingly, although Everglades hydrology has flow as one of its defining characteristics, most discussions of hydrology in the Everglades exclude mention of the role of water movement. The movement of water in aquatic ecosystems such as wetlands is a fundamental construct of ecosystem structure and function, and its ecosystem role is well- established. It is likely that water movement plays a similar vital role in the Everglades.
The ridge and slough landscape, one of several major habitat types in the Everglades, originally consisted of a peat-based system of dense sawgrass ridges interspersed with adjacent and relatively open sloughs. These parallel ridges and sloughs existed in an organized pattern, oriented parallel to the flow direction, on a slightly sloping peatland. Unfortunately, compartmentalization and related water management activities are resulting in the loss of this ridge and slough landscape. This loss is evidenced by replacement of the characteristic ridge and slough landscape with a landscape that is more topographically and vegetationally uniform. It is clear that 1) the Everglades ridge and slough landscape has changed, and is continuing to change significantly; and 2) the landscape changes are having detrimental ecological effects on Everglades plants and animals. It is likely that these changes are the result of altered water flow and hydropattern caused by human-made barriers
and shunts, interacting with corresponding changes in water depth and water level fluctuations.
The mechanisms causing the loss of ridge and slough landscape likely are complex, are occurring over a time scale of decades or more, and restoration decisions will have to be made on a time frame shorter than decades. A number of mechanisms for the formation and maintenance of the ridge and slough landscape are proposed, including: sediment transport; changes in water depth under managed conditions; differential rates of peat accumulation and decomposition; erosional formation; extreme hydrological events; fire; underlying bedrock patterns; and microhabitat differences in water chemistry. The presence of flow is necessary for almost all of these mechanisms to operate, and it is likely that a combination of several of these mechanisms operating together is responsible for formation and maintenance of the ridge and slough landscape.
The mechanisms of ridge and slough landscape degradation are not fully understood. However, it is likely that barriers to flow, including levees and canals, contribute significantly to the conversion of the ridge and slough wetland mosaic to more uniform stands of sawgrass. Conversion of the ridge and slough landscape pattern to uniform sawgrass stands has had, and will continue to have, deleterious impacts on Everglades plants and animals. An Everglades landscape increasingly dominated by dense sawgrass stands supports fewer numbers of animals and a lower diversity of animals. The control of vegetation over wading bird ecology is strong enough that Kushlan (1989) states, “Whatever determines vegetation patterns will also, to a large degree, determine bird use of wetlands.” Wading birds are an important component of the Everglades ecosystem. Their foraging and nesting success often are used as indicators of the overall health of the system, and they are one of the most visible and highly regarded fauna of the Everglades. The conversion of ridge and slough landscape to dense sawgrass stands has had a negative impact on wading birds and other important birds of the Everglades. Negative impacts of these landscape changes extend throughout the Everglades food web, including fish, which are important food for wading birds. In addition to altering flow patterns and wetland landscape patterns, barriers to flow serve as barriers to movement of aquatic animals.
Very few research studies have been conducted specifically to determine the role of flow in the Everglades ecosystem. Most of the research projects from which data are presented in this paper, while relevant to the role of flow, were not designed to determine the role of flow. It is precisely for this reason that the Science Coordination Team chose the topic of flow as one of its priorities. Recommendations for future research are prioritized, and include: a multidisciplinary paleoenvironmental study; a thorough geomorphic review; sediment transport studies; synoptic and time series measurements of flow; development of a carbon balance model; remote sensing; and others.
Finally, additional performance measures are recommended for the ridge and slough landscape for the monitoring and assessment of the Comprehensive Everglades Restoration Plan (CERP) progress. These measures, largely based on expanded collection and analysis of remotely sensed images, include: aerial extent and temporal trends of sawgrass ridge, slough, and tree island polygons; edge-to-area ratios for landscape types; average length-to-width ratio and temporal trends of sawgrass ridges or sloughs for a defined area; and spatial orientation of the three landscape types as compared to their historic orientations.