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

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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.

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

References (69)

  • A. Piqué

    The geological evolution of Madagascar: an introduction

    J. Afr. Earth Sci.

    (1999)
  • A. Piqué et al.

    Extension in Madagascar from the Neogene to the present: structural and geophysical data

    J. Afr. Earth Sci.

    (1999)
  • N.A. Wells et al.

    The initiation and growth of gullies in Madagascar: are humans to blame?

    Geomorphology

    (1993)
  • Aldegheri, M., 1972. Rivers and streams on Madagascar. In: Battistini, R., Richard-Vindard, G. (Eds.), Biogeography and...
  • F. Arthaud

    Mise en Evidence d’une neotectonique en distension N–S a Madagascar (Hauts Plateaux)

    C.R. Acad. Sci., Series 2

    (1989)
  • F. Arthaud et al.

    La Tectonique Cassante a Madagascar; Son Incidence sur la Geomorphologie et sur les Ecoulement

    Can. J. Earth Sci.

    (1990)
  • Baron, R., 1890. A Malagasy forest. Antananarivo Annual 4 (14) 196–211. In: Kull, C.A. (Ed.), Isle of Fire. The...
  • Baron, R., 1891. The flora of Madagascar. Antananarivo Annual 4 (15) 322–357. In: Kull, C.A. (Ed.), Isle of Fire. The...
  • Battistini, R., 1972. Madagascar relief and main types of landscape. In: Battistini, R., Richard Vindard, G., Junk, W....
  • Battistini, R., Verin, P., 1972. Man and the Environment in Madagascar: past problems and problems of today. In:...
  • Besairie, H., 1964. Carte Géologique de Madagascar, Antananarivo: Service Géologique de Madagascar, scale: 1:1,000,000,...
  • H. Besairie

    Carte geologique au 1:2,000,000 et notice explicative

    Docum. Bur. Geol. Madagascar

    (1971)
  • H. Besairie

    Madagascar

    Pure Appl. Geophys.

    (1971)
  • H. Besairie

    Carte Geologique au 1/200000 et notice explicative, Ser. Geol. Madagascar, Antananarivo

    Pure Appl. Geophys.

    (1973)
  • D.B. Botkin et al.

    Environmental Science, Earth as a Living Planet

    (2003)
  • Bourgeat, F., 1972. Sols sur Socle Ancien a Madagascar. Type de Differenciation et Interpretation Chronologique au...
  • Bourgeat, F., 1979. The genesis and general morphology of soils of Madagascar. In: Rev. Geogr. Madagascar, issue 35,...
  • Bourgeat, F., 1996. Les Grandes Unites Pedo-Morphologiques dans la Region de Morondova. In: Ganzhorn, J.U., Sorg, J.P....
  • Brenon, P., 1949. Etude Geologique de la Feuille Lac Alaotra. In: Travaux du Bureau Geologique, Republique Malagasy,...
  • Brenon, P., 1956. Le Graben du Lac Alaotra, Madagascar. In: Publication-Scientific Council for Africa South of the...
  • Burgis, M.J., Symoens, J.J., 1987. African wetlands and shallow water bodies: directory. ORSTOM, Paris. In: Goodman,...
  • Burney, D.A., 1997. Theories and facts regarding Holocene environmental change before and after human colonization. In:...
  • Burney, D.A., 2003. Madagascar’s prehistoric ecosystems. In: Goodman, S.M., Benstead, J.P. (Eds.), The Natural History...
  • Cox, R., Rakontondrazafy, A.F.M., Rakontondramazava, H.T., 2004. Geological versus human controls on lavaka formation...
  • Dewar, R.E, 1997. Were people responsible for the extinction of Madagascar subfossils, and how will we ever know? In:...
  • M. de Wit

    Madagascar: heads it’s a Continent, tails it’s an Island

    Ann. Rev. Earth Planet. Sci.

    (2003)
  • Dudan, R., Boulanger, J., Bertucat, M., Clair, G., 1959. Levers: Carte Geologique de Lac Alaotra (coupure speciale),...
  • Dudan, R., Boulanger, J., Bertucat, M., Clair, G., 1961. BRGM: Carte Geologique de Lac Alaotra (coupure speciale),...
  • J.M. Dufils

    Forest ecology

  • Du Puy, D.J., Moat, J., 2003. Using geological substrate to identify and map primary vegetation types in Madagascar and...
  • Durbin, J., Rakotoniaina, L.J., Randriamahefasoa, J., 2003. Project Alaotra: using endangered species as flagships for...
  • F.T.M (Foiben-Taosarintanin’i Magagasikara), 1978. Carte Topographique d’Ambohijanahary, Feuille S-43, Echelle 1/100...
  • G.W. Gade

    Deforestation and its effects in highland Madagascar

    Mount. Res. Develop.

    (1996)
  • Gezon, L.L., Freed, B.Z., 1999. Agroforestery and conservation in Northern Madagascar: hopes and hinderances. Afr....
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