Canals and Levees and Everglades Ecology
South Florida’s ecosystem centers on the Everglades. Historically, Lake Okeechobee, Big Cypress, the ridge and slough Everglades, and mangrove coastal swamps were all linked and formed a sheet of shallow water that flowed south across the landscape. Major changes came to the system in the 20th century when canals and levees were dredged and built to combat flooding and provide water for urban and agricultural areas. These drainage efforts substantially lowered the Everglades’s water levels and altered the north to south flow. Today, Everglades management attempts to balance ecology with the needs of south Florida’s agriculture and urban population.
Canals and Levees
Canals have crossed the Everglades since pre-Columbian times, but its modern canals are wider, deeper, and hundreds of kilometers longer than any that existed previously. Because of their size and prominence, canals have shaped and changed the area’s ecology, water movement, and recreational options.
The canal and levee system simultaneously cuts-off and connects the landscape. It can be a barrier to surface water flow and wildfires while increasing nutrient and pollutant spread and retention. In terms of wildlife movement, canals and levees help or hinder depending on the species.
As the amount of research on the Everglades system increases, it is becoming clear that the structure of canals and levees and the division of the landscape have degraded the Everglades ecosystem. In 2000, the U.S. Congress authorized the Comprehensive Everglades Restoration Plan (CERP) to restore natural water flow while also providing water supplies for south Florida’s farms and cities. The CERP has made removing and modifying canals and levees in the Everglades Protection Area (EPA) its central priority. The categories below describe the interaction of the EPA’s ecological and environmental factors with the canal and levee system.
Levees store water during dry periods and prevent flooding over low-lying areas. Canals drain wetlands, which opens land for development and increases water delivery to cities, wells, and the coast. The main effects of canals and levees on Everglades water movement have been:
- Wetlands loss and lowering of water tables
- Reduced freshwater flow and increased salinity in estuaries
- Depletion of natural water storage areas
- Water delivery shift from slow sheet flow to “pulses” through canals
- Low flow pools that are too deep for diverse plant vegetation
- Complete dry-down during the dry season that leads to diminished aquatic habitat and flattening of peat surface
- Canal intrusion into the aquifer and increased salinity and groundwater-surface water interaction
Canals and levees change the Everglades from a relatively stable shallow water environment to fragmented extremes of too wet (deep water) and too dry areas.
Canals deposit nutrient runoff, especially phosphorous, into the wetlands. The natural algae and vegetation mix evolved under nutrient poor conditions, and the increased nutrients are changing plant communities from sawgrass to cattail dominated—this in turn changes oxygen levels and fish populations.
Research of canal sediment and surface water has found pesticides at every sampling site. DDT and methylmercury are among the various herbicides and insecticides.
Canals and levees interrupt and break up the historical continuous wetlands and sheet flow of the Everglades system. Water shaped the Everglades, so changing the flow and availability of water automatically changes the landscape.
The current Everglades landscape does not have the size, seasonal pattern, or habitat variety and connectivity to support its historic populations of wildlife. Dividing the Everglades has degraded the traditional slough-and-ridge and tree-island landscape and blocked environmental processes, such as seasonal flooding and wildfires, from moving through the region.
Canals enable exotic fish species to survive in the Everglades by providing permanent refuges from drought and cold weather. Recent monitoring determined that up to seventy percent of the fish in canals can be nonnative. Canals also act as pathways for the dispersal of invasive snails and clams.
The pest plants water lettuce, hydrilla, and water hyacinth find the deep-water, nutrient-rich habitat they need in Everglades canals. Their dense vegetation outcompetes native plants and also hurts navigation, flood control, and recreation use in the canals.
Levees create upland habitat that allows Australian pine, Brazilian peppertree, and exotic grasses to grow and provides sites for fire ant and Burmese python nests.
Canals as wildlife habitat is a complex topic that is still being researched, but here are some ideas of ways canals change habitats:
Without canals, many species would not be able to move into interior wetlands. Canals open up wetlands not only for exotic species, but also native fish, as canals support movement and range expansion within the region. While both native and exotic fish numbers have increased in canals, the question is how many actually move out of the canals into the wetlands.
There are seasonal changes to wildlife populations in canals. During the dry season, canals and deep water pools serve as refuges for aquatic species, especially predators like bass and alligators. Canals provide either permanent or seasonal habitat for species populations to grow when wetlands conditions are not ideal.
The Everglades bass fishery is a product of the canal system. Canals enhance recreational fishing through the combination of nutrient enrichment, increased prey population during the dry season, and low habitat complexity (which allows predators to be more efficient).
Alligators have moved out of the marshes into canals. These alligators no longer build or maintain alligator holes that provide wetland dry season habitat for fish, amphibians, and bird species. Increased alligator populations in canals also leads to higher predation on prey species, flooded alligator nests along canal shorelines, and adult alligator domination in canals.
Manatees use the canals as alternative habitat sites, especially during winter months when they seek out warmer waters. They often are trapped and killed by locks and water control gates, the second greatest reason for manatee death caused by human activity.
Canals and levees are needed for south Florida’s water management system, but they have had multiple negative effects on the Everglades Protection Area. Removing and modifying these structures (decompartmentalization) in the EPA would benefit the ecosystem by:
- Restoring wetland sheet flow and decreasing rapid canal routing and drainage of water
- Reconnecting wetlands separated by canal and levee barriers
- Eliminating artificial habitats that support and spread introduced plants and animals in the Everglades
The CERP’s restoration plan calls for removing interior canals and levees, while adding more on the periphery of the Everglades. Cooperation of managers and engineers with ecologists and hydrologists will hopefully lead this project in the direction of giving careful consideration to ecological impacts and developing a sustainable method of delivering water.
To view maps and find more detailed information on research sources and proposed system modifications, read the source document Effects of Canals and Levees on Everglades Ecosystems.
Adapted and excerpted from:
Rebecca Harvey, et al, Effects of Canals and Levees on Everglades Ecosystems (WEC304), Wildlife Ecology and Conservation Department (12/2010).