Everglades landscape evolution: islands, ridges, and channels

The Florida Everglades is a vast wetland mosaic of sloughs, ridges, and tree islands, with extremely small hydraulic gradients and low relief. Elongated ridges of peat separated by sloughs in the regional flow direction form a strikingly rhythmic landscape pattern. Many parts of the Everglades have seen extensive peat loss and ecological change due to drainage and other hydraulic controls over the last century. A suite of numerical models has been developed to examine interactions between surface-water and groundwater flow, vegetation, peat dynamics, nutrient transport, and anthropogenic hydraulic controls. These models are supported by a set of field experiments and observations, and explanations for the landscape patterns have begun to arise from this interaction. Organic peat soil oxidizes rapidly when dry, whereas peat formation is a slow process controlled by water levels, vegetative turnover, and possibly scour and organic soil transport. Formation and maintenance of raised peat levels in the face of oxidation, separated by lower sloughs, is partially explained by net phosphorus transfer from slough to ridge during dry-season evapotranspiration-driven subsurface lateral flow, which helps concentrate the limiting nutrient (phosphorus) in higher ground, thus enhancing plant growth and fueling more evapotranspiration. Plume-like tree islands on and downstream of local bedrock rises may have phosphorus export due to phosphorus mining and selective deposition of guano in addition to experiencing the evapotranspiration-driven phosphorus enrichment process. Although historic records are scanty, there is anecdotal evidence of sharp transitions between peat ridges and exposed limestone channel bottoms. Exposed-bedrock channels between peat beds may be the result of water levels gradually rising as peat forms, leaving sufficient water depth in low-lying areas to scour clean the deeper channels without affecting the shallower peat. Numerical simulations demonstrate the plausibility of the postulated ridge and channel formation processes.