Disappearing Lake Alaotra: Monitoring catastrophic erosion, waterway silting, and land degradation hazards in Madagascar using Landsat imagery
Introduction
Madagascar is located in the southern hemisphere, 400 km from western coast of Africa from which it separated 160 Million years ago forming the Mozambique Channel (de Wit, 2003) and later from India around 70 Ma. Dense forest that once covered the island has been dramatically reduced by many factors. Deforestation is initially attributed to climate changes both global and local in scale (Grainger, 1993), and also was influenced by the human influences including clear-cutting, burning, farming, logging, and construction of new dwellings. Results of deforestation in eastern Madagascar are dramatic. For instance, the largest lake on the island, Lake Alaotra, has shrunken to a small fraction of its former size. Lake Alaotra is located in a N–S striking tectonic depression, bounded by highlands on the west and east, and characterized by active faulting, thermal springs, nearby Neogene volcanics. East–west extension caused by the separation of India–Madagascar in Late Cretaceous (Harland et al., 1990) generated a Neogene graben (Brenon, 1956, Besairie, 1971a, Besairie, 1971b, Besairie, 1973, de Wit, 2003) called the Lake Alaotra basin, that has generally north–south trending faults and fractures (Arthaud, 1989, Laville, 1998) that probably control the recharge of the lake.
This study focuses on the recent geologic history of Lake Alaotra, which is located 750 m above sea level in the Ambatondrazaka area in eastern Madagascar (Fig. 1). It is the biggest lake on the island and the surrounding area is used for rice-cultivation and is regarded as the most productive area on the island (e.g. Wright and Rakotoarisoa, 2003). Unfortunately, due to the climate changes and dramatic deforestation (Dufils, 2003), the lake has been dramatically reduced in recent years; its size and its shape have changed and the rice production has decreased. Towns such as Ambatondrazaka, once on the lakeshore, are now located tens of kilometers from the water. Our purpose in this study is to document the shrinkage of Lake Alaotra, the reduction of the dense primary forest and the evolution of erosion in a gap of particular time.
Combined Landsat, and historical studies reveal that the dramatic clear-cutting of surrounding forest has crossed an erosional threshold triggering denudational processes causing catastrophic degradation of hill slopes by gullies called lavakas. Hill slopes have been rapidly eroded, shedding soil that covers low-lying areas with silts. Streams that once flowed deep are now braided shallow anastomosing channels. Muddy and silty waters covering rice patties have reduced rice-cultivation, and Lake Alaotra is being rapidly filled with sediment. Lake Alaotra is now only 20–30% of its primary size, and at times appears to totally disappear.
Section snippets
Geologic setting
Lake Alaotra is located in eastern Madagascar centered between 17th and 18th degree of south latitude, and 48th and 49th degree of east longitude at about 750 m above the sea level (F.T.M, 1978), and its area is recognized as a basin defined by faults on its eastern and western sides. The basin is surrounded by hills, which rise around 900–1300 m above the sea level (Dudan et al., 1959, Dudan et al., 1961). Precambrian crystalline rocks, predominantly migmatitic rocks of the graphitic system of
Methods
To investigate the relationships between deforestation, land degradation, and silting of waterways we integrate use of satellite remote sensing imagery taken several years apart, and field observations in and around the Lake Alaotra basin. Satellite images including Landsat 7 ETM + (2000 data) and Landsat 7 TM (1993 data) (Table 1) are employed to make band combinations and band ratio images showing the changing distinction of the area covered and uncovered by forests. We also highlight the
Results
Lake Alaotra was once several times larger than its present size (Moreau, 1987). The size of Lake Alaotra is not steady; it varies from around 35–40 km long and 5–9 km wide within 50 years (Table 2). From this table, the size of Lake Alaotra varies; input of water depends on climatic condition mainly during rainy season.
The basin is classified as a wetland area (Durbin et al., 2003). It is made up of the clear water surface (Lake Alaotra) surrounded by marshy zone of Cyperus Emyrnensis (zozoro)
Discussion
Madagascar was once a green island (covered by forest), and only recently become a red island (red lateritic land exposed on the hillside by erosional process and colored streams and rivers into red) (Le Bourdiec, 1972). Yet, seeing grassland in the highland and the western part of Madagascar, many explorers (e.g. Battistini and Verin, 1972) asked “did the forest cover the whole or part of the island in the time?” According to Perrier de la Bâthie (1921), the isolated residual forest within
Conclusion
In conclusion, many factors contribute to the shrinkage of Lake Alaotra including climate changes since Neogene period, including primary deforestation starting in the last century (burning, clear-cutting, logging, farming and new space clearing for dwellings). The lake is now only 20–30% of its primary size. About 5 km2 is the lost of lake surface within 30 years. The recharge of the Lake Alaotra is no longer great enough to fill it to its former size. Now, the infiltration of the incoming
Acknowledgements
This work was supported by National Science Foundation grants 01-25925, 02-21567, and 02-07997, awarded to Dr. T. Kusky. We greatly thank all thoughtful reviewer comments including R. Cox and M. Frei for significant improvement and suggestion of the manuscript. I also particularly owe thanks to my family for spending their time to provide documents with help of Mr. A. Randrianasolo to bring these important documents in U.S. We are also thankful for additional information received from many
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