How is Everglades geomorphology like that of arid Australian rivers and boreal bogs?
Shortcut URL: https://serc.carleton.edu/69109
Continent: North America
Country: United States
City/Town: Miami/Ft. Lauderdale
UTM coordinates and datum: 26.056521°, -80.705348°, WGS84
Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, fluvial, modeling,
Clues from patterns
Geormorphic patterns are interesting not just because of the sense of wonder they evoke, but because they provide evidence of a delicate balance created by feedback. If that feedback is disrupted by perturbations in climate or other drivers, that landscape may undergo a rapid and dramatic shift to an alternate morphology. Understanding how patterns form can provide insight into the perturbations they are most sensitive to and how these landscapes should be managed to best conserve form and function.
Degradation of Everglades landscape pattern
The strikingly patterned Everglades ridge and slough landscape (Fig. 1) is one example of a system that has undergone rapid change as its hydrology has become more managed. Across a 50-kilometer wide swath, this flowing, freshwater part of the Everglades was originally differentiated into regularly spaced, ~100-m-wide by kilometer-long peat ridges that were colonized by dense sawgrass and lower, more open sloughs. Elevation differences between the two features is presently only ~10 cm; in the past, it may have just been several tens of centimeters. Current trajectories of landscape change are making the patterning more subtle; elevation differences between ridges and sloughs are diminishing, and throughout much of its extent, sawgrass from ridges has been filling in sloughs.
Everglades landscape changes are a product of the recent past. Pollen in peat cores evidences spatial invariance in the position of ridges since the time of their formation out of a dominantly slough environment two millennia ago. Although ridges expanded and contracted with regional climatic fluctuations (the wet Medieval Warm Period and drier Little Ice Age, respectively), only during the last century were sloughs lost so rapidly and completely.
What changed to promote this shift? Since around 1900, the Everglades experienced a host of anthropogenic perturbations: drainage for flood control and water supply, compartmentalization of the formerly free-flowing "River of Grass" with levees, introduction of invasive exotic species, and contamination with agricultural nutrients. But which of these factors were changes in the ridge and slough landscape responding to?
Disruptions in flow seemed to be a likely candidate. One of the most unique and intriguing features of the ridge and slough landscape is its orientation parallel to flow. Classic geomorphology predicts that parallel-drainage networks are stable only on steeply sloped terrain; at moderate to low gradients, randomly greater erosion rates in one of the flowing channels initiates a positive feedback that reinforces erosion within that channel at the expense of the other channels, leading to discharge capture and the formation of a dominant channel. Why, then, did the parallel-drainage ridge and slough landscape appear to be a stable morphology for millennia, and could recent disruptions in flow have triggered its Anthropocene instability?
Anabranching streams: an Everglades analog?
At the time the Comprehensive Everglades Restoration Plan was authorized by the US Congress, scientists had little understanding of the feedbacks producing landscape patterning in the Everglades. However, the Everglades had a potential analog in another type of persistent low-gradient parallel-drainage landscape: ridge-form anabranching streams (Fig. 2), found in arid central Australia, the Indian subcontinent, and several other locations worldwide. Anabranching streams are distinguished from braided streams—another multi-threaded channel system—in that channel positions are relatively persistent, and islands separating channels aggrade to the level of the floodplain. Typically found in environments with excessive sediment supply relative to the energy available to transport sediment and with high seasonality in flow, anabranching streams would begin to develop when sediment was deposited on the streambed during high-flow events. During the intervening dry periods, vegetation would colonize the incipient ridges and stabilize them. Subsequently, flow separation around the ridges under flood conditions would promote ridge elongation, and incipient ridges would grow together. The bank stabilization provided by the vegetation roots would enable channels to achieve lower width-to-depth ratios, which contributed to their empirically greater sediment transport capacity compared to single-threaded reaches and moving the river system toward an equilibrium between sediment supply and transport capacity.
Testing the role of a sediment redistribution feedback in the Everglades
Simulation modeling (Fig. 3) suggests that similar processes guided early landscape formation in the Everglades. Within a peat-based, organic system, microbes and higher plants in the Everglades are continually producing a large supply of flocculent, organic sediment composed of partially decomposed detrital material. With limited flow, that "floc" moves gradually downgradient at the edges of ridges and contributes to the outward growth of ridges. However, as ridges grow outward, they concentrate more flow within sloughs, improving their transport capacity. At some point, flow at the ridge edges is sufficient to entrain and transport enough sediment at ridge edges to balance sediment additions from ridges, and the system becomes laterally stable (Fig. 4).
A peat accretion feedback confers vertical stability
Sediment eroded from sloughs and ridge edges is dominantly deposited on ridge tops further downstream, so why is it that sloughs and ridges historically maintained a relatively constant elevation difference? The answer lies in a second feedback process—one that also causes hummocks and hollows to maintain stable, consistent elevation differences in boreal bogs. Although the tall, dense vegetation characteristic of hummocks and ridges initially produces organic sediment at a more rapid rate than that of sloughs or hollows, as ridges grow toward the water surface, they become more aerated, and decomposition proceeds at a faster rate. At some elevation, organic matter accumulation on ridges and sloughs becomes equivalent. When sediment deposition locally causes ridges to become higher, even faster rates of decomposition will result in a decrease in elevation back to the equilibrium point (Fig. 5).
Widely occurring feedback processes confer clues for restoration management
Scientists now think that it is this latter peat accretion feedback process that responds most rapidly to anthropogenic change. Diminishing water levels have reset the equilibrium elevation of ridges, causing them to become shorter but also wider as more portions of ridge edges have been exposed. But flow still needs to be sufficient to erode sediment from ridge edges to prevent gradual spreading of ridges and declining slough connectivity over time.
Although by first appearances a unique landscape, the feedbacks controlling Everglades geomorphology are nonunique, shared with environments as diverse as boreal bogs and arid-zone rivers. Through simulation model-aided synthesis, awareness of those feedbacks in other systems has led to breakthroughs in our understanding of the Everglades, with positive implications for restoration management.
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