The Case of the Vanishing Marsh
The Report Team at Work

It was a task that could puzzle the shrewdest of sleuths and perplex the brightest of brains— identify precisely the parts of Louisiana’s coast that are already missing, and then predict which areas will disappear in the next 50 years.

Fourteen experts in remote sensing technologies, geographic information systems and ecosystem processes accepted the challenge of detecting land loss in Louisiana. Representing nine academic institutions and government agencies, this team used satellite imagery, aerial photography, maps and previous land-change studies to calculate the number of square miles lost in the past 25 years, and to project those losses into the future.

Detecting What’s Missing

To figure the amount of land already lost, the team classified all cartographic points as either land or water. By comparing data from the same location in different years, they determined which points had converted from land to water, or from water to land. Their measurements indicated that, since 1932, Louisiana has suffered a net loss of 1,900 square miles. Although loss rates peaked in the 1970s, between 1978 and 2000 Louisiana continued to lose land at a rate of 29.9 square miles a year.

Erosion Accelerates as Shorelines Expand

Estimate of combined shoreline, or land-water interface, peaks about 2002, after which land area decreases, interface shrinks, and land loss rates slow.
(Based on graph by Browder, et al. 1985, 1989)

“At first glance, this decline might look like good news,” says Rex Caffey, professor at the LSU Agricultural Center and Louisiana Sea Grant, “but it really shows how dire the situation is. If you think of the marshes as a huge slab of Swiss cheese floating in the water, land erodes along the inside edges of the holes. As the holes grow larger and connect, there is more water and less edge, less interface between land and water.” As a consequence, the rate of loss declines even though conditions may not have improved. A graph describing this phenomenon, known as the Browder curve, indicates that the maximum amount of land-water interface in coastal Louisiana might have occurred in 2002. Since then, the shrinkage of land area has itself caused the rate of loss to fall. “Rather than indicating that the problem is less severe,” Caffey says, “the declining rate of loss shows us how much damage we’ve suffered already.”

Fish Populations Increase, Then Collapse, as Marshes Erode

a. Erosion occurs along the edge of marsh, where fish find food and habitat.

b. As marsh erodes and breaks apart, edge perimeter increases and fish populations multiply.

c. As eroded marsh vanishes, edge perimeter declines and fish populations collapse.

S. Coogle, Koupal Communications,
based on diagram in Louisiana Sportsman

Foretelling the Vanishing Points

After a careful analysis, the report team concluded that it could use the conditions present between 1990 and 2000 as a baseline for the next 50 years. The team divided a map of the coast into areas that shared a similar cause, extent and pattern of land change, and compared shifts in each set of circumstances to discern trends in land change. Combining this information with the baseline data, the team predicted the likely location and degree of future land loss, estimated the amount of possible land gain and calculated the balance of expected change. The team concluded that, if no further action is undertaken to slow the loss, at least 513 more square miles of Louisiana land will vanish by the year 2050. The total amount of land lost since 1932 would then exceed 2,400 square miles, reducing the coastal area of the state by a third in a little over 100 years.

In declining wetlands, the perimeters of small segments of
marsh increase the total measure of land-water borderlines.
Courtesy of Louisiana Office of Tourism