Earthshots: Satellite Images of Environmental Change

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» The Everglades and Miami, Florida, USA

Referred to as the “River of Grass,” the Florida Everglades is one of the world’s largest wetlands. However, they were once much larger. Throughout the 20th century, the region was drained for agriculture and development, and canals and levees were built to manage the water. The area that is protected as a national park, established in 1947, is a fraction of the original extent of the Everglades.

The Everglades is essentially a wide, slow-moving sheet of shallow water, hence the name River of Grass. The water originates at the Kissimmee River in central Florida, drains into Lake Okeechobee, then out into the Everglades. A rich variety of plant and animal habitats occupy this very flat terrain—even small water level changes significantly impact these communities.

Much of the development in the Everglades started long before the Space Age, but notable land changes occurred within the satellite era. This series of Landsat images shows the urban development of the Miami metropolitan area, agriculture, and the extent of land protections of southeastern Florida.

Map of the featured area.

Historically, water flowed slowly southward through the Everglades in a wide swath. Record floods in 1947 and 1948 led to the construction of a massive flood control project. It served to prevent flooding and store water during dry periods. It also allowed for further development of the growing urban area on the Atlantic coastal ridge.

The project established three Water Conservation Areas (WCAs), one of which is the Loxahatchee National Wildlife Refuge. These areas are delineated in the Landsat images, clearly divided by the levees and canals. Also visible are the Big Cypress National Preserve, Everglades National Park, and Everglades Agricultural Area.

Another part of the project is the 100-mile-long eastern perimeter levee, a 3- to 6-meter high earthen berm built to prevent flooding of farmland and urban areas. It runs along the eastern edge of the WCAs, marking a clear separation between the WCAs and urban areas such as Miami, Fort Lauderdale, and Coral Springs.

Along with facilitating the further growth of the urban areas of greater Miami, the other upshot of the project was that the natural flow of water was interrupted, changing the hydrology of the region. The gradual sheet flow of freshwater is diminished, and instead sudden pulses of water are delivered by the canals. These sudden releases caused decreases in the numbers of fish species.

A unique feature of the Everglades is the dark green teardrop-shaped forms scattered throughout the region. Referred to as hummocks, or “tree islands,” these biodiversity hotspots provide food, cover, and critical nesting sites for numerous species.

The patches of woody vegetation range in size from 0.01 to 70 hectares, and they stand out from the sawgrass and marsh landscape 0.6–1.2 meters (2–4 feet) above the slough bottom. The teardrop shape generally points in the direction of the flow of water.

Tree islands can have as much as 10 times the nutrient phosphorus as the surrounding Everglades. They soak up both phosphorus and nitrogen, allowing these spots to flourish and provide habitats for plants, invertebrates, amphibians, reptiles, mammals, and resident and migratory birds, increasing overall biodiversity. As these islands store up phosphorus, they keep phosphorus low in the surrounding marsh. When the phosphorus concentrations in the rest of the marsh goes up, cattail invades, which affects fish and wading bird populations.

The number and areal extent of tree islands have been reduced over the past 75 years with the changes in hydrology caused by the levees and canals.

The fertile muck soils along the south shore of Lake Okeechobee in the Everglades Agricultural Area are intensively farmed. Tomatoes, beans, squash, peppers, and other crops are grown during the winter and shipped north. Sugarcane is the main crop there from spring through the fall.

The bottom center of the later images reveals the location of Stormwater Treatment Area (STA) 3/4, one of six STAs mandated by the State of Florida’s Everglades Forever Act, passed in 1994. STA 3/4 is distinct from the polygons of the agricultural area. Just to its west is Holey Land Wildlife Management Area.

These constructed wetlands are designed to remove phosphorus that comes from agricultural runoff. Under previous agricultural practices, phosphorus from fertilizers was allowed to run off into the Everglades. Phosphorus is an essential nutrient, but too much of it alters the Everglades’ natural habitats.

These treatment areas channel runoff through shallow marshes filled with plants that absorb phosphorus, reducing the amount that flows into the Everglades. The plants include cattail, southern naiad, hydrilla, and algae. These plants continue to absorb phosphorus even after they die and decompose. The underlying limestone layer then holds the phosphorus, providing long-term storage.

Besides helping to restore the Everglades, these treatment areas have become a great home for wading birds, ducks, and alligators. Florida offers hunting and other recreational opportunities at many of these treatment areas.

In 1900, southeastern Florida included a few small towns with a population of 3,592 people. Throughout the 20th century, Miami and other nearby towns steadily grew. According to the U.S. Census Bureau, the population of the Miami Metropolitan Statistical Area in 2016 was 6,066,387. In 1970, its population was listed as 1,267,792. In four and one-half decades, the population increased by over 378%.

The urban area of Miami and surrounding suburbs stand out sharply against the dark green wetlands in the Landsat series. The urban areas lie on a slightly higher ridge, 2 to 7 meters above sea level, to form a “backbone” of the heavily urbanized coast.

These images show urban development expanding on agricultural land north and west of Homestead. Homestead’s population grew from 13,674 in 1970 to 66,498 in 2015, an increase of 386%.

There is a notable shape southeast of Homestead on the coast of the Bay of Biscayne. The top right corner of this shape is the location of the Turkey Point Nuclear Plant. In operation since 1972, the plant generates enough power for 900,000 homes annually.

The rest of the large area is the plant’s system of cooling canals. Water in this closed loop of canals is used to keep equipment in the plant cool. The water passes through the plant to remove excess heat then goes back into the canal system. The water cools as it travels through the canals.

Landsat’s thermal infrared imaging capability reveals the temperature of the water in this system. The brighter red in that image indicates warmer water. The water becomes cooler away from the plant and coolest on the right side nearest the plant where the water reenters the plant.

Other warmer surfaces appear in a red hue as well, such as highways, airport runways, and other less vegetated surfaces of Homestead.

A lot of the urban area consists of large planned communities, often for retirees. Many of these communities incorporate artificial residential lakes. Digging out these lakes provided construction fill for roads and elevated low-lying land for development. The lakes also reduce the risk of urban flooding by capturing storm water runoff, and add aesthetic value.

Comparing the images in this time series, the number of small water bodies increases substantially. The small dark shapes are scattered throughout the green shades that indicate residential areas, along with golf courses, which appear brighter green. The dark green in the lower left is the Loxahatchee National Wildlife Refuge.

An image from Sentinel-2 at 20-meter resolution shows even more clearly these residential lakes, golf courses, and suburban areas.

The Everglades sit on top of a bedrock of limestone. Limestone quarries, the dark rectangular shapes just west of the urban areas, provide about half of the rock used in Florida’s construction.

In this part of Florida, groundwater is very near the surface. So as the rock is mined, the quarry pits fill with water. These lakes range from 30 to 50 feet deep and cover a total of about 4,900 acres. Because of these artificial lakes, the region earned the name “The Lake Belt.” These rectangular water bodies expand and change shape over time in the image series.

Near the top center to the left of the highway, some of the areas that started out as mines turned into developed, with that desirable waterfront incorporated into the residential areas.

The prominent diagonal line running from the upper left to the lower right is the Miami Canal, which flows from Lake Okeechobee and into the Miami River near the airport. The river then flows through downtown Miami and into Biscayne Bay.

On the far left of these images is a vertical line that marks the boundary of the Francis S. Taylor Wildlife Management Area. Between that and the limestone pits is the Pennsuco Wetlands, part of the east coast buffer project, designed to act as a zone of protection between the Everglades and the urban centers further east. Water can be captured, stored, and released when it benefits both the urban communities and the ecosystem. Higher water levels in the Everglades then have less chance to impact the populated areas.


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