Disappearing Lake Alaotra: Monitoring catastrophic erosion, waterway silting, and land degradation hazards in Madagascar using Landsat imagery

Abstract

At the turn of the century the island of Madagascar was densely forested, but it has recently been dramatically deforested with most damage since French colonization of the island in 1896. When the French arrived many Malagasy people fled into the forest to survive and practiced deforestation to obtain land for cultivation. However, limited slash and burn had already been an historical practice even before French colonization. Over 90% of the Malagasy original forest is now gone. Severe environmental issues have arisen on the island caused by this deforestation including degradation of the landscape due to erosion followed by siltation of many streams and rivers, and loss of Madagascar’s biodiversity. Only the humid eastern and northeastern forests remain in significant amounts and even these are concentrated in areas of steep slope and difficult access. Madagascar’s largest lake, Lake Alaotra, is located at about 750 m elevation along the eastern escarpment, in the east-central part of the country. The lake is in a large fault-controlled basin and is known for the islands most fertile and productive rice fields. However, in the past 30 years, silt derived from erosion of the soil horizon, a consequence of the deforestation, has clogged the streams and rivers in the Lake Alaotra Basin, and has filled in most of the lake. Examination of sequential Landsat TM imagery, along with field and historical observations, has shown that the lake had shrunk to 60% of its former size by the 1960s, 40% of its former size by the 1980s, and 20% of its former size in 2000. Fieldwork in the lake basin in 2003 revealed that Lake Alaotra was essentially gone, leaving only small swampy areas and perhaps a shallow small remnant lake and surrounded by marsh transformed into rice-cultivation. Crop productivity in the basin has also dropped dramatically to about 40% of its former level as a consequence of the silting of the rivers and irrigation canals, yet clear-cutting and slash and burn clearing continues in the basin. The disappearance of Lake Alaotra and the loss of crop productivity are the result of the environmental degradation. Image analysis and GIS modeling is used to locate areas that are contributing the largest amount of silt to the basin so remedial action can be taken to reduce further loss and crop yield degradation, but no matter what is done, Lake Alaotra has filled with silt and will not easily return.